Physics Final Review Mr. Miller
Determine the quantity and type of charge on an object that has 3.62 x 1012 more protons than electrons. (static electricity)
+5.8 x 10^-7 Coulombs (rounded) To determine the charge on an object, determine the number of excess protons or excess electrons. Multiply the excess by the charge of an electron or the charge of a proton - 1.6 x 10-19 C. Finally, adjust the sign of the object to + or -.
Two charged objects have an attractive force of 0.080 N. If the distance separating the objects is quadrupled, then what is the new force?
0.0050 N Explanation: The electrostatic force is inversely related to the square of the separation distance. So if d is four times larger (quadrupled), then F is 16 times smaller - that is, 1/16-th the original value. One-sixteenth of 0.080 N is 0.0050 N.
Two charged objects have a repulsive force of 0.080 N. If the distance separating the objects is tripled, then what is the new force?
0.00889 N Explanation: The electrostatic force is inversely related to the square of the separation distance. So if d is three times larger, then F is nine times smaller - that is, one-ninth the original value. One-ninth of 0.080 N is 0.00889 N.
Two charged objects have a repulsive force of 0.080 N. If the distance separating the objects is doubled, then what is the new force?
0.020 N Explanation: The electrostatic force is inversely related to the square of the separation distance. So if d is two times larger, then F is four times smaller - that is, one-fourth the original value. One-fourth of 0.080 N is 0.020 N.
Two charged objects have an attractive force of 0.080 N. If the charge of one of the objects is tripled and the distance separating the objects is tripled, then what is the new force?
0.0267 N Explanation: The electrostatic force is directly related to the product of the charges and inversely related to the square of the separation distance. Tripling one of the charges would serve to triple the force. Tripling the distance would serve to reduce the force by a factor of nine. The combined affect of these two variations would be to make the force 3/9-ths or 1/3-rd the original value. One-third of 0.080 N is 0.0267 N.
At what distance of separation must two 1.00-microCoulomb charges be positioned in order for the repulsive force between them to be equivalent to the weight (on Earth) of a 1.00-kg mass?
0.030 m or 3.0 cm Step 1: Identify known values in variable form. Q1 = 1.0 x 10-6 C and Q2 = 1.0 x 10-6 C Felect = Fgrav = mg = 1.0 • 9.8 m/s/s = 9.8 N Step 2: Identify requested information d = ??? Step 3: Substitute and solve. (See example C on the Tutorial page for sample algebra.)
Two charged objects have a repulsive force of 0.080 N. If the charge of one of the objects is doubled, and the distance separating the objects is doubled, then what is the new force?
0.040 N Explanation: The electrostatic force is directly related to the product of the charges and inversely related to the square of the separation distance. Doubling one of the charges would serve to double the force. Doubling the distance would serve to reduce the force by a factor of four. The combined affect of these two variations would be to decrease the force by a factor of two - changing it from 0.080 N to 0.040 N.
A positively charged object with a charge of +85 nC is being used to balance the downward force of gravity on a 1.8-gram balloon that has a charge of -63 nC. How high above the balloon must the object be held in order to balance the balloon? (NOTE: 1 nC = 1 x 10-9 C)
0.052 meters Given: Q1 = +8.5 x 10-8 C Q2 = -6.3 x 10-8 C m = 0.0018 kg Use the mass to determine the force of gravity (m • g). The force of gravity on the balloon is 0.0176 N. Thus, the upward electrical force is 0.0176 N. Now that F, Q1, and Q2 are known, Coulomb's law can be used to determine the distance d in the equation. Algebraic rearrangement leads to d = Sqrt [ (k• Q1• Q2) / F ]. Substitution leads to the answer.
Two charged objects have a repulsive force of 0.080 N. If the charge of both of the objects is doubled and the distance separating the objects is doubled, then what is the new force?
0.080 N Explanation: The electrostatic force is directly related to the product of the charges and inversely related to the square of the separation distance. Doubling both of the charges would serve to quadruple the force. Doubling the distance would serve to reduce the force by a factor of four. The combined affect of these two variations would be to not change the force at all; it remains as 0.080 N.
Two charged objects have an attractive force of 0.080 N. If the charge of one of the objects is increased by a factor of four, and the distance separating the objects is doubled, then what is the new force?
0.080 N Explanation: The electrostatic force is directly related to the product of the charges and inversely related to the square of the separation distance. Quadrupling one of the charges would serve to quadruple the force. Doubling the distance would serve to reduce the force by a factor of four. The combined affect of these two variations would be to not alter the force at all; it would remain as 0.080 N.
a hunter uses a blow gun to hunt for supper. a force of 2.0 newtons is applied to a 0.05 kg dart for 0.75 seconds. the speed of the dart as it leaves the blow gun is about: (momentum and impulse)
0.13 m/sec
Joann has rubbed a balloon with wool to give it a charge of -1.0 x 10-6 C. She then acquires a plastic golf tube with a charge of +4.0 x 10-6 C localized at a given position. She holds the location of charge on the plastic golf tube a distance of 50.0 cm above the balloon. Determine the electrical force of attraction between the golf tube and the balloon.
0.144 N Step 1: Identify known values in variable form. Q1 = -1.0 x 10^-6 C and Q2 = +4.0 x 10-6 C d = 50.0 cm = 0.50 m. Step 2: Identify requested information F = ??? Step 3: Substitute and solve. (See example B on the Tutorial page for sample algebra.)
Two charged objects have a repulsive force of 0.080 N. If the charge of one of the objects is doubled, then what is the new force?
0.160 N Explanation: Electrostatic force is directly related to the charge of each object. So if the charge of one object is doubled, then the force will become two times greater. Two times 0.080 N is 0.160 N.
A balloon with a charge of 4.0 µC is held a distance of 0.70 m from a second balloon having the same charge. Calculate the magnitude of the repulsive force.
0.29 N Step 1: Identify known values in variable form. Q1 = -4.0 x 10^-6 C and Q2 = +4.0 x 10-6 C d = 0.70 m. Step 2: Identify requested information F = ??? Step 3: Substitute and solve. (See example B on the Tutorial page for sample algebra.)
Two charged objects have a repulsive force of 0.080 N. If the charge of both of the objects is doubled, then what is the new force?
0.320 N Explanation: Electrostatic force is directly related to the charge of each object. So if the charge of both objects is doubled, then the force will become four times greater. Four times 0.080 N is 0.320 N.
Two charged objects have a repulsive force of 0.080 N. If the distance separating the objects is halved, then what is the new force?
0.320 N Explanation: The electrostatic force is inversely related to the square of the separation distance. So if d is two times smaller, then F is four times larger. Four times 0.080 N is 0.320 N
Determine the electrical force of attraction between two balloons that are charged with the opposite type of charge but the same quantity of charge. The charge on the balloons is 6.0 x 10-7 C and they are separated by a distance of 0.50 m.
1.3 x 10-2 N, attractive (rounded from 1.296 x 10-2 N) Step 1: Identify known values in variable form. Q1 = +6.0 x 10-7 C and Q2 = -6.0 x 10-7 C d = 0.50 m Step 2: Identify the requested value F = ??? Step 3: Substitute and solve (See example B on the Tutorial page for sample algebra.)
a mass with a momentum of 40 kg*m/sec receives an impulse of 30 n*sec in the direction opposite to its motion. its final momentum is: (momentum and impulse)
10 kg*m/sec
a 2 kilogram piece of clay moving at 3 m/sec strikes and sticks to a second 2 kilogram piece of clay moving at 1 m/sec in the opposite direction. Calculate the speed of the combined piece of clay. (momentum and impulse)
2 m/sec
Determine the electrical force of attraction between two balloons with separate charges of +3.5 x 10-8 C and -2.9 x 10-8 C when separated a distance of 0.65 m.
2.16 x 10-5 N, attractive Step 1: Identify known values in variable form. Q1 = +3.5 x 10-8 C and Q2 = -2.9 x 10-8 C d = 0.65 m Step 2: Identify the requested value F = ??? Step 3: Substitute and solve (See example B on the Tutorial page for sample algebra.)
the impulse necessary to cause a change of momentum of 20 kg*m/sec for a 5 kg object is: (momentum and impulse)
20 n*sec
the time required to stop a 200 kilogram wagon moving at 5 m/sec with a 40 newton force is: (momentum and impulse)
25 seconds
eighty percent of the mass of a 20,000 kg rocket is fuel. if this fuel is ejected as exhaust gases with an average velocity of 1 km/sec with respect to the ground, the final velocity of the rocket will be: (momentum and impulse)
250 km/sec
Balloon A and Balloon B are charged in a like manner by rubbing with animal fur. Each acquires an excess of 25 trillion electrons. If the mass of the balloons is 1 gram, then how far below Balloon B must Balloon A be held in order to levitate Balloon B? Assume the balloons act as point charges.
3.83 meters Begin by determining the Q of the balloons. Q1 = Q2 = # of excess electrons • Qelectron = 4.0 x 10-6 C. The force of gravity of the balloons is m • g or 0.0098 N. To levitate the top balloon over the bottom balloon, the electrical force of repulsion must equal the force of gravity on the top balloon. Thus Felect = 0.0098 N. Now that Q1, Q2, and F are known, Coulomb's law can be used to determine the distance d. Algebraic rearrangement leads to d = Sqrt [ (k• Q1 • Q2 ) / F ]. Substitution leads to the answer.
what is the momentum of a 30-kg cart moving at a speed of 10 meters per second? (momentum and impulse)
300 kg*m/sec
The amount of charge carried by a lightning bolt is estimated at 10 Coulombs. What quantity of excess electrons is carried by the lightning bolt? (static electricity)
6.25 x 1019 electrons Multiply the charge in Coulombs (10 C) by the conversion factor: (1 electron) / (1.6 x 10-19 C). The units of C cancel; the answer is in electrons.
during a football game, a 90 kg running back moving east is tackled by a 110 kg defensive lineman running west at 5 m/sec. both players move east at 1.5 m/sec after the tackled is made. before he is tackled the running back is moving with a speed of about: (momentum and impulse)
6.7 m/sec
Two 1.2-gram balloons are suspended from light strings attached to the ceiling at the same point. The net charge on the balloons is -540 nC. The balloons are distanced 68.2 cm apart when at equilibrium. Determine the length of the string.
78.8 cm Like the example problem above, it is best to begin with a sketch of the situation and a free-body diagram. Q1 and Q2 are known to be -5.4x10-7 C. The separation distance is 0.682 m. Using Coulomb's law and values of Q1, Q2, and d, the electric force can be found to be 5.64x10-3 N. The force of gravity is m • g or 0.0118 N. From the FBD and the sketch, one sees that the tangent(theta) = Felect / Fgrav = (5.64x10-3 N) / (0.0118 N) = 0.4798. Thus, theta (the angle with the vertical) is 25.6 degrees. From a distance triangle, one sees that sin(theta) = 0.5 • d / L Substituting theta and d into this equation leads to the answer.
A rubber balloon possesses a positive charge. If brought near and touched to the door of a wooden cabinet, it sticks to the door. This does not occur with an uncharged balloon. These two observations can lead one to conclude that the wall is _____. a. electrically neutral b. negatively charged c. a conductor d. lacking electrons
A Observing the positively charged balloon stick to the wall is evidence that the wall is either neutral or charged negatively. Once the second observation is made, one can conclude that the wall is neutral. If it were charged, then the wall and the uncharged balloon should attract.
A positively charged balloon is brought near a neutral conducting sphere as shown below. While the balloon is near, the sphere is touched (grounded). At this point, there is a movement of electrons. Electrons move ____ . a. into the sphere from the ground (hand) b. out of the sphere into the ground (hand) c. into the sphere from the balloon d. out of the sphere into the balloon e. from the ground through the sphere to the balloon f. from the balloon through the sphere to the ground g. .... nonsense! Electrons do not move at all.
A Since the balloon is not contacted to the sphere, electrons do NOT move between the balloon and the sphere (ruling out choices c, d, e, and f). The presence of the positive balloon draws electrons from ground to the sphere. This is the principle of opposites attract.
Two objects are shown at the right. One is neutral and the other is negative. Object X will ____ object Y. a. attract b. repel c. not affect
A X is charged and Y is neutral. Charged and neutral objects always attract each other.
Balloons X , Y and Z are suspended from strings as shown at the right. Negatively charged balloon X attracts balloon Y and balloon Y attracts balloon Z. Balloon Z ____. List all that apply. a. may be positively charged b. may be negatively charged c. may be neutral d. must be positively charged e. must be negatively charged f. must be neutral
A, B and C Y is observed to attract a negatively charged object (balloon X). So Y could be either positively charged or neutral. Y attracts Z. If Y were neutral (and we don't know for sure that it is), then Z would attract Y if it were either + or -. So A and B are two possible answers. But Y could be positively charged. And if Y were positively charged, the Y-Z attraction would be observed if Z were neutral. So choice C is also possible.
Explain the connection between your answers to the above questions and the reasoning used to explain why an object moving in a circle at constant speed can be said to experience an acceleration. (Circular Motion and Satellite Motion: Acceleration)
An object which experiences either a change in the magnitude or the direction of the velocity vector can be said to be accelerating. This explains why an object moving in a circle at constant speed can be said to accelerate - the direction of the velocity changes.
When an oil tanker car has arrived at its destination, it prepares to empty its fuel into a reservoir or tank. Part of the preparation involves connecting the body of the tanker car with a metal wire to the ground. Suggest a reason for why is this done.
As fuel is pumped from the tanker car to a reservoir, charge can quickly build up as the fluid flows through the hoses. This static charge can create sparks capable of igniting the fuel. By connecting the body of the tanker car to the ground, the static charge can be transferred to the ground. A metal wire is used since metals are conductive and allow charge to flow through them.
Two objects are charged as shown at the right. Object X will ____ object Y. a. attract b. repel c. not affect
B Both C and Y are charged with excess + charge. Having the same type of charge, they will repel.
A positively charged pop can is touched by a person standing on the ground. The pop can subsequently becomes neutral. The pop can becomes neutral during this process because ______. a. electrons pass from the pop can to the person (ground) b. electrons pass from the person (ground) to the pop can c. protons pass from the pop can to the person (ground) d. protons pass from the person (ground) to the pop can
B Protons do NOT move during electrostatic activities, so choices c and d can be ruled out. To ground a positively charged object, electrons must be added to it in order neutralize its excess positive charge. So electrons must move from the ground into the pop can.
A positively charged piece of Styrofoam is placed on the table. A neutral aluminum pie plate is brought near as shown below. While held above the Styrofoam, the aluminum plate is touched (grounded). At this point, there is a movement of electrons. Electrons move ____ . a. out of the aluminum plate into the ground (hand) b. into the aluminum plate from the ground (hand) c. into the aluminum plate from the Styrofoam d. out of the aluminum plate into the Styrofoam e. from the ground through the aluminum plate to the Styrofoam f. from the Styrofoam through the aluminum plate to the ground g. .... nonsense! Electrons do not move at all.
B Since the foam is not contacted to the aluminum plate, electrons do NOT move between the foam and the aluminum (ruling out choices c, d, e, and f). The presence of the positively charged foam plate draws electrons from ground to the aluminum plate. This is the principle of opposites attract.
TRUE or FALSE? Two neutral conducting pop cans are touching each other. A negatively charged balloon is brought near Can X as shown below. As the balloon approaches Can X, there is a movement of electrons between the balloon and can X (in one direction or the other).
B - False In induction charging, there is never a transfer of electrons between the charged object (the balloon) and the object being charged (Can X). The electron movement happens between the object being charged (Can X) and the ground (Can Y). In this case, electrons would leave Can X and enter Can Y.
In the above situation, the conducting sphere is ____. List all that apply. a. charged b. uncharged (neutral) c. polarized
B and C The conducting sphere is both uncharged (there is a balance between the number of + and - charges) and polarized (there is a separation of the + from the - charges).
Which of the following materials are likely to exhibit more conductive properties than insulating properties? _____ Explain your answers. a. rubber b. aluminum c. silver d. plastic e. wet skin
B, C and E Aluminum and silver are metals, making them good conductors. The human body is a fairly good conductor. When wet, its an even better conductor.
A physics teacher rubs a glass object and a felt cloth together and the glass becomes positively charged. Which of the following statements are true? Circle all that apply. a. The glass gained protons during the rubbing process. b. The felt became charged negatively during this rubbing process. c. Charge is created during the rubbing process; it is grabbed by the more charge-hungry object. d. If the glass acquired a charge of +5 units, then the felt acquires a charge of -5 units. e. This event violates the law of conservation of charge. f. Electrons are transferred from glass to felt; protons are transferred from felt to glass. g. Once charged in this manner, the glass object and the felt cloth should attract each other. h. In general, glass materials must have a greater affinity for electrons than felt materials.
B, D, G For A: protons are not transferred in electrostatics. For C: charge is neither created nor destroyed (ever). For E: electrons are simply transferred, consistent with the law of conservation of charge. For F: protons are not transferred in electrostatic experiments (the electrons are transferred). For H: If glass transfers electrons to felt, then the felt must be more electron-hungry.
A neutral metal sphere is touched by a negatively charged metal rod. As a result, the sphere will be ____ and the metal rod will be ____. Select the two answers in their respective order. a. positively charged b. negatively charged c. neutral d. much more massive e. ... not enough information to tell
BB This is a case of charging by conduction. When a charged object is used to charge a neutral object by conduction, the previously neutral object acquires the same type of charge as the charged object. The charge object maintains the same type of charge that it originally had. So in this case, both objects have a negative charge.
A metal sphere is electrically neutral. It is touched by a positively charged metal rod. As a result, the metal sphere becomes charged positively. Which of the following occur during the process? List all that apply. a. The metal sphere gains some protons.,/p. b. Electrons are transferred from the sphere to the rod. c. The metal sphere loses electrons. d. The overall charge of the system is conserved. e. Protons are transferred from the rod to the sphere. f. Positive electrons are moved between the two objects.
BCD In electrostatic activities, protons are never transferred (which rules out choices a and e). Electrons are not positively charged (ruling out choice e). Choices B, C and D are all true and explain the essential nature of the conduction charging process.
3. Before beginning its initial descent, a roller coaster car is always pulled up the first hill to a high initial height. Work is done on the car (usually by a chain) to achieve this initial height. A coaster designer is considering three different incline angles at which to drag the 2000-kg car train to the top of the 60-meter high hill. In each case, the force applied to the car will be applied parallel to the hill. Her critical question is: which angle would require the most work? Analyze the data, determine the work done in each case, and answer this critical question. (Work, Energy, and Power: Calculating the Amount of Work Done by Forces ) Angle Force Distance Work (J) a. 35 deg 1.12 x 104 N 105 m Work? b. 45 deg 1.39 x 104 N 84.9 m Work? c. 55 deg 1.61 x 104 N 73.2 m Work:
Be careful! The angle in the table is the incline angle. The angle theta in the equation is the angle between F and d. If the F is parallel to the incline and the d is parallel to the incline, then the angle theta in the work equation is 0 degrees. For this reason, W=F*d*cosine 0 degrees. In each case, the work is approximately 1.18 x106 Joules. The angle does not affect the amount of work done on the roller coaster car.
Which statement best explains why a rubber rod becomes negatively charged when rubbed with fur? a. The rubber that the rod is made of is a better insulator than fur. b. The fur is a better insulator than the rubber. c. Molecules in the rubber rod have a stronger attraction for electrons than the molecules in the fur. d. Molecules in the fur have a stronger attraction for electrons than the molecules in the rubber rod.
C During a charging by rubbing (or by friction) process, the material that becomes negatively charged does so because it simply likes electrons more than the material with which it is rubbed. The conductive abilities of the two materials has nothing to do with the subsequent result of the rubbing process.
A neutral metal sphere is touched by a negatively charged metal rod. During the process, electrons are transferred from the _____ to the _____ and the sphere acquires a _____ charge. a. neutral sphere, charged rod, negative b. neutral sphere, charged rod, positive c. charged rod, neutral sphere, negative d. charged rod, neutral sphere, positive e. ... nonsense! None of these describe what occurs.
C During charging by conduction, both objects acquire the same type of charge. If a negative object is used to charge a neutral object, then both objects become charged negatively. In order for the neutral sphere to become negative, it must gain electrons from the negatively charged rod.
Electrical forces ____. (static electricity) a. can cause objects to only attract each other b. can cause objects to only repel each other c. can cause objects to attract or repel each other d. have no effect on objects
C Electric forces are repulsive for objects of like charge and attractive between objects of the opposite type of charge or between charged objects and neutral objects.
the quantity electric potential is defined as the amount of _____. a. electric potential energy b. force acting upon a charge c. potential energy per charge d. force per charge
C Electric potential is the amount of potential energy per unit of charge.
Suppose that a conducting sphere is charged positively by some method. The charge is initially deposited on the left side of the sphere. Yet because the object is conductive, the charge spreads uniformly throughout the surface of the sphere. The uniform distribution of charge is explained by the fact that ____. a. the charged atoms at the location of charge move throughout the surface of the sphere b. the excess protons move from the location of charge to the rest of the sphere c. excess electrons from the rest of the sphere are attracted towards the excess protons
C Rule out A since atoms are not capable of moving within solid spheres. Rule out B since protons are not capable of moving in electrostatic demos. C is the proper explanation since the negative electrons are attracted to the region of positive charge. The electrons migrate towards the left side of the sphere until there is a uniform distribution of positive charge.
Moving an electron within an electric field would change the ____ the electron. a. mass ofb. amount of charge onc. potential energy of
C When a force is required to move an electron in the direction of an electric field, its electrical potential energy increases. On the other hand, an electron moving opposite the direction of the electric field will decrease its electrical potential energy. This is because the electric field direction is in the direction which a positive charge spontaneously moves. An electron is negatively charged.
ean Yuss is investigating the charge on several objects and makes the following findings. object c= attracts B Object D= repels C Object E= attracts D, repels F Object F= attracts A Jean knows that object A is negatively charged and object B is electrically neutral. What can Jean Yuss definitively conclude about the charge on objects C, D, E, and F? Explain.
C and D are -; E and F are + It's best to start on the right side of the table. Observing the E-F repulsion is enough to conclude that both E and F MUST be charged with like charge. Since F and A attract, the charge on F is +; and thus the charge on E must also be +. The C-D repulsion is sufficient evidence to conclude that both C and D MUST be charged. Since D and E attract, D must have the opposite charge of E. E has been declared as +, so D MUST be -. If C has like charge as D, it must be - also.
Upon entering the room, you observe two balloons suspended from the ceiling. You notice that instead of hanging straight down vertically, the balloons seems to be repelling each other. You can conclusively say ... a. both balloons have a negative charge. b. both balloons have a positive charge. c. one balloon is charge positively and the other negatively. d. both balloons are charged with the same type of charge.
D Observing a repulsive interaction is sufficient evidence to conclude that both balloons are charged. However, further testing or additional information would be required to determine the type of charge the balloons have.
If an electrical circuit were analogous to a water circuit at a water park, then the battery voltage would be comparable to _____. a. the rate at which water flows through the circuit b. the speed at which water flows through the circuit c. the distance that water flows through the circuit d. the water pressure between the top and bottom of the circuit e. the hindrance caused by obstacles in the path of the moving water
D The battery establishes an electric potential difference across the two ends of the external circuit and thus causes the charge to flow. The battery voltage is the numerical value of this electric potential difference. In an analogous manner, it is the difference in water pressure between the top of the water slide and the bottom of the water slide that the water pump creates. This difference in water pressure causes water to flow down the slide. Because of the similarity between electric potential difference in an electric circuit and water pressure in a water park, the quantity electric potential difference is sometimes referred to as electric pressure.
During a physics lab, a plastic strip was rubbed with cotton and became positively charged. The correct explanation for why the plastic strip becomes positively charged is that ... a. the plastic strip acquired extra protons from the cotton. b. the plastic strip acquired extra protons during the charging process. c. protons were created as the result of the charging process. d. the plastic strip lost electrons to the cotton during the charging process.
D When two different materials are rubbed together, there is a transfer of electrons from one material to the other material. This causes one object to become positively charged (the electron loser) and the other object to become negatively charged (the electron gainer).
On many occasions, there is more than one force acting upon an object. A free-body diagram is a diagram that depicts the type and the direction of all the forces acting upon an object. The following descriptions and their accompanying free-body diagrams show the forces acting upon an object. For each case, indicate which force(s) are doing work upon the object. Then calculate the work done by these forces. Diagram 1: A 10-N force is applied to push a block across a friction free surface for a displacement of 5.0 m to the right. Diagram 2: A 10-N frictional force slows a moving block to a stop after a displacement of 5.0 m to the right. Diagram 3: A 10-N force is applied to push a block across a frictional surface at constant speed for a displacement of 5.0 m to the right. Diagram 4: An approximately 2-kg object is sliding at constant speed across a friction free surface for a displacement of 5 m to the right. Diagram 5: An approximately 2-kg object is pulled upward at constant speed by a 20-N force for a vertical displacement of 5 m. Work, Energy, and Power: Calculating the Amount of Work Done by Forces
Diagram 1: Forces Doing Work on the Object: Only Fapp does work. Fgrav and Fnorm do not do work since a vertical force cannot cause a horizontal displacement. Amount of Work Done by Each Force: Wapp= (10 N) * (5 m) *cos (0 degrees) = +50 Joules Diagram 2: Forces Doing Work on the Object: Only Ffrict does work. Fgrav and Fnorm do not do work since a vertical force cannot cause a horizontal displacement. Amount of Work Done by Each Force: Wfrict =(10 N) * (5 m) * cos (180 degrees) = -50 Joules Diagram 3: Forces Doing Work on the Object: Fapp and Ffrict do work. Fgrav and Fnorm do not do work since a vertical force cannot cause a horizontal displacement. Amount of Work Done by Each Force: Wapp = (10 N) * (5 m) * cos (0 deg) = +50 Joules Wfrict = (10 N) * (5 m) * cos (180 deg) = -50 Joules Diagram 4: Forces Doing Work on the Object: Neither of these forces do work. Forces do not do work when they makes a 90-degree angle with the displacement. Amount of Work Done by Each Force: No work is done. Diagram 5: Fgrav and Ftens do work. Forces do work when there is some component of force in the same or opposite direction of the displacement. Amount of Work Done by Each Force: Wtens = (20 N) * (5 m) * cos (0 deg) = +100 Joules Wgrav = (20 N) * (5 m) * cos (180 deg) = -100 Joules
Apply the work equation to determine the amount of work done by the applied force in each of the three situations described below. Diagram A: A 100N force is applied to move a 15 kg object a horizontal distance of 5 meters at constant speed. Diagram B: A 100N force is applied at an angle of 30 degrees to the horizontal to move a 15 Kg object at a constant speed for a horizontal distance of 5 m. Diagram C: An upward force is applied to lift a 15 kg object to a height of 5 meters at constant speed. Work, Energy, and Power: Calculating the Amount of Work Done by Forces
Diagram A Answer: W = (100 N) * (5 m)* cos(0 degrees) = 500 J The force and the displacement are given in the problem statement. It is said (or shown or implied) that the force and the displacement are both rightward. Since F and d are in the same direction,the angle is 0 degrees. Diagram B Answer: W = (100 N) * (5 m) * cos(30 degrees) = 433 J The force and the displacement are given in theproblem statement. It is said that the displacement is rightward. It is shown that the force is 30 degrees above the horizontal. Thus, the angle between F and d is 30 degrees. Diagram C Answer: W = (147 N) * (5 m) * cos(0 degrees) = 735 J The displacement is given in the problem statement. The applied force must be 147 N since the 15-kg mass (Fgrav=147 N) is lifted at constant speed. Since F and d are in the same direction, the angle is 0 degrees.
The Q in Coulomb's law equation stands for the _____. a. mass of a charged object b. # of excess electrons on the object c. the current of a charged object d. the distance between charged objects e. charge of a charged object
E In the equation Felect = k • Q1 • Q2 / d2 , the symbol Felect represents the electrostatic force of attraction or repulsion between objects 1 and 2. The symbol k is Coulomb's law constant (9 x 109 N • m2 / C2), Q1 and Q2 represent the quantity of charge on object 1 and object 2, and d represents the separation distance between the objects' centers.
A neutral metal sphere is touched by a positively charged metal rod. During the process, protons are transferred from the _____ to the _____ and the sphere acquires a _____ charge. a. charged rod, neutral sphere, negative b. charged rod, neutral sphere, positive c. neutral sphere, charged rod, negative d. neutral sphere, charged rod, positive e. ... nonsense! None of these describe what occurs.
E Protons are never transferred in electrostatic activities. In this case, electrons are transferred from the neutral object to the positively charged rod and the sphere becomes charged positively.
Respond to the following student statement: (static electricity) "A positively charged object is an object that has an excess of positive electrons."
Electrons are not positively charged. Positively charged objects have an excess of protons (which are positively charged).
A conductor differs from an insulator in that a conductor ________. a. has an excess of protons b. has an excess of electrons c. can become charged and an insulator cannot d. has faster moving molecules e. does not have any neutrons to get in the way of electron flow f. none of these
F A and B are characteristic of positive and negative objects. As for C, both insulators and conductors can be charged. As for D, this has nothing to do with the conductive properties of materials. As for E, neutrons are located in the nucleus and are "out of the way" of mobile electrons.
A physics student, standing on the ground, touches an uncharged plastic baseball bat to a negatively charged electroscope. This will cause ___. a. the electroscope to be grounded as electrons flow out of the electroscope. b. the electroscope to be grounded as electrons flow into the electroscope. c. the electroscope to be grounded as protons flow out of the electroscope. d. the electroscope to be grounded as protons flow into the electroscope. e. the baseball bat to acquire an excess of protons. f. absolutely nothing (or very little) to happen since the plastic bat does not conduct.
F In order to ground an electroscope, electrons must have a conducting pathway between the ground and the object. In this case, a piece of plastic is part of the pathway connecting the ground (the student) and the charged object. Since plastic is an insulator, electrons are incapable of moving through the baseball bat. Grounding does not occur in this instance. Were there a conducting pathway available, choice a would be the proper choice.
TRUE or FALSE: An object that becomes grounded gains neutrons during the grounding process.
False Neutrons are positioned in the nucleus of an atom. And like protons, neutrons are never transferred in electrostatic experiments. They are bound in the nucleus and cannot escape by ordinary electrostatic methods.
True or False: When an object becomes polarized, it acquires a charge and becomes a charged object.
False When an object becomes polarized, its center of positive charge becomes separated from its center of negative charge. Overall, there are just as many positive charges as negative charges; the object has a balance of charges and is therefore neutral.
The symbol d in Coulomb's law equation represents the distance from ___. a. A to B b. A to D c. B to C d. B to D e. C to D f. A to G g. B to F h. C to E
G In the equation Felect = k • Q1 • Q2 / d2 , the symbol Felect represents the electrostatic force of attraction or repulsion between objects 1 and 2. The symbol k is Coulomb's law constant (9 x 109 N • m2/ C2), Q1 and Q2 represent the quantity of charge on object 1 and object 2, and d represents the separation distance between the objects' centers.
Calculate the work done by a 2.0-N force (directed at a 30° angle to the vertical) to move a 500 gram box a horizontal distance of 400 cm across a rough floor at a constant speed of 0.5 m/s. (HINT: Be cautious with the units.) Work, Energy, and Power: Calculating the Amount of Work Done by Forces
Here is a good example of the importance of understanding the angle between F and d. In this problem, the d is horizontal and the F is at a 60-degree angle to the horizontal. Thus, theta is 60 degrees. W = (2.0 N) * (4.00 m) * cos (60 degrees) = 4.0 J
If the roller coaster car in the above problem were moving with twice the speed, then what would be its new kinetic energy? Work, Energy, and Power: Kinetic Energy
If the speed is doubled, then the KE is quadrupled. Thus, KE = 4 * (1.04653 x 105 J) = 4.19 x 105 Joules. or KE = 0.5*m*v2 KE = 0.5*625 kg*(36.6 m/s)2 KE = 4.19 x 105 Joules
Missy Diwater, the former platform diver for the Ringling Brother's Circus, had a kinetic energy of 12 000 J just prior to hitting the bucket of water. If Missy's mass is 40 kg, then what is her speed? Work, Energy, and Power: Kinetic Energy
KE = 0.5*m*v2 12 000 J = (0.5) * (40 kg) * v2 300 J = (0.5) * v2 600 J = v2 v = 24.5 m/s
Determine the kinetic energy of a 625-kg roller coaster car that is moving with a speed of 18.3 m/s. Work, Energy, and Power: Kinetic Energy
KE = 0.5*m*v2 KE = (0.5) * (625 kg) * (18.3 m/s)2 KE = 1.05 x105 Joules
A 900-kg compact car moving at 60 mi/hr has approximately 320 000 Joules of kinetic energy. Estimate its new kinetic energy if it is moving at 30 mi/hr. (HINT: use the kinetic energy equation as a "guide to thinking.") Work, Energy, and Power: Kinetic Energy
KE = 80 000 J The KE is directly related to the square of the speed. If the speed is reduced by a factor of 2 (as in from 60 mi/hr to 30 mi/hr) then the KE will be reduced by a factor of 4. Thus, the new KE is (320 000 J)/4 or 80 000 J.
Complete the following statements: (static electricity) After some rather exhausting counting (and a rather tall tale), a physics teacher determines that a very small sample of an object contains ... a. ... 8.25749 x 1017 protons and 5.26 x 1014 electrons; the charge on this object is ____ Coulombs. b. ... 3.12 x 1014 protons and 4.5488 x 1016 electrons; the charge on this object is ____ Coulombs. c. ... 2.40277 x 1019 protons and 9.88 x 1016 electrons; the charge on this object is ____ Coulombs. d. ... 2.6325 x 1015 protons and 2.6325 x 1015 electrons; the charge on this object is ____ Coulombs.
Method: Subtract the smaller number from the larger number. (This would be based upon the exponent.) Then multiply the difference by the charge of a proton or electron - 1.6 x 10-19 C. Answers: a. 0.132 C b. 0.00723 C (7.23 x 10-3 C) c. 3.83 C d. 0 Coulombs
A cart is loaded with a brick and pulled at constant speed along an inclined plane to the height of a seat-top. If the mass of the loaded cart is 3.0 kg and the height of the seat top is 0.45 meters, then what is the potential energy of the loaded cart at the height of the seat-top? Work, Energy, and Power: Potential Energy
PE = m*g*h PE = (3 kg ) * (9.8 m/s/s) * (0.45 m) PE = 13.2 J
Saran Wrap has a larger electron affinity than Nylon. If Nylon is rubbed against Saran Wrap, which would end up with the excess negative charge? ____________ Explain.
Saran Wrap When two materials are rubbed together, the material with the greatest affinity for electrons is the material which takes electrons away from the other material. Saran wrap takes electrons from nylon and acquires the negative charge. In turn, the nylon loses electrons and becomes charged positively.
Ben Travlun carries a 200-N suitcase up three flights of stairs (a height of 10.0 m) and then pushes it with a horizontal force of 50.0 N at a constant speed of 0.5 m/s for a horizontal distance of 35.0 meters. How much work does Ben do on his suitcase during this entire motion? Work, Energy, and Power: Calculating the Amount of Work Done by Forces
The motion has two parts: pulling vertically to displace the suitcase vertically (angle = 0 degrees) and pushing horizontally to displace the suitcase horizontally (angle = 0 degrees). For the vertical part, W = (200 N) * (10 m) * cos (0 deg) = 2000 J. For the horizontal part, W = (50 N) * (35 m) * cos (0 deg) = 1750 J. The total work done is 3750 J (the sum of the two parts).
A tired squirrel (mass of 1 kg) does push-ups by applying a force to elevate its center-of-mass by 5 cm. Estimate the number of push-ups that a tired squirrel must do in order to do a approximately 5.0 Joules of work. Work, Energy, and Power: Calculating the Amount of Work Done by Forces
The squirrel applies a force of approximately 10 N (9.8 N to be exact) to raise its body at constant speed. The displacement is 0.05 meters. The angle between the upward force and the upward displacement is 0 degrees. The work for 1 push-up is approximately W = 10 N * 0.05 m * cos 0 degrees = 0.5 Joules If the squirrel does a total of 5.0 Joules of work, then it must have done about 10 push-ups.
A student with a mass of 80.0 kg runs up three flights of stairs in 12.0 sec. The student has gone a vertical distance of 8.0 m. Determine the amount of work done by the student to elevate his body to this height. Assume that his speed is constant. Work, Energy, and Power: Calculating the Amount of Work Done by Forces
The student weighs 784 N (Fgrav= 80 kg * 9.8 m/s/s). To lift a 784-Newton person at constant speed, 784 N of force must be applied to it (Newton's laws). The force is up, the displacement is up, and so the angle theta in the work equation is 0 degrees. Thus, W = (784 N) * (8 m) * cos (0 degrees) = 6272 Joules
If a force of 14.7 N is used to drag the loaded cart (from previous question) along the incline for a distance of 0.90 meters, then how much work is done on the loaded cart? Work, Energy, and Power: Potential Energy
W = F * d * cos Theta W = 14.7 N * 0.9 m * cos (0 degrees) W = 13.2 J
A force of 50 N acts on the block at the angle shown in the diagram. The block moves a horizontal distance of 3.0 m. How much work is done by the applied force? (30 degrees) Work, Energy, and Power: Calculating the Amount of Work Done by Forces
W = F * d * cos(Theta) W = (50 N) * (3 m) * cos (30 degrees) = 129.9 Joules
which of the following has the greatest momentum? (momentum and impulse) a tractor with a mass of 2350 kg moving at a velocity of 26 m/sec? a boat with a mass of 1310 kg moving at a velocity of 53 m/sec? a truck with a mass of 6220 kg moving at a velocity of 11 m/sec? a car with a mass of 1440 kg moving at a velocity of 39 m/sec?
a boat with a mass of 1310 kg moving at a velocity of 53 m/sec?
a collision between two object referred to as elastic would be characterized by: (momentum and impulse)
a loss of kinetic energy and no permanent change in shape
Identify the following particles as being charged or uncharged. If charged, indicate whether they are charged positively or negatively. (n = neutron, p = proton, e = electron) (static electricity) Particle A= 8e outside 2e inside 10 neutrons 9 protons Particle B= 1 Electrons second outer layer 8 electrons outer layer 2 electrons inside 12 neutrons inside 11 protons Particle C= 8 electrons second outer layer 8 electrons outer layer 2 electrons inside 20 neutrons inside 20 protons inside
a. Charged Negatively There are 11 electrons and 10 protons. This results in an imbalance of charge. With more electrons than protons, the particle is negatively charged. b. Uncharged There are 11 electrons and 11 protons. This results in a balance of charge. This particle is neutral or uncharged. c. Charged Positively There are 18 electrons and 20 protons. This results in an imbalance of charge. With more protons than electrons, the particele is positively charged.
you throw a basketball forward while standing on a skateboard. this causes: (momentum and impulse)
backward motion of the board and you
Two neutral conducting pop cans are touching each other. A positively charged balloon is brought near one of the cans as shown below. The cans are separated while the balloon is nearby, as shown. After the balloon is removed the cans are brought back together. When touching again, can X is ____. a. positively charged b. negatively charged c. neutral d. impossible to tell
c. Neutral When the balloon is near, electrons leave Can Y and enter Can X. Overall, the two cans are neutral; yet as individual cans, Can X is negatively charged and Can Y is positively charged. When the cans are touched again, the excess electrons in Can Y return to Can X. Once more, the overall charge on the system of two cans is zero - the system is neutral.
____ are the charged parts of an atom. (Static Electricity) . Only electrons b. Only protons c. Neutrons only d. Electrons and neutrons e. Electrons and protons f. Protons and neutrons
electrons and protons
TRUE or FALSE: An object that is electrically neutral contains only neutrons. (static electricity)
false
TRUE or FALSE: An object that is negatively charged could contain only electrons with no accompanying protons. (static electricity)
false
TRUE or FALSE: An object that is positively charged contains all protons and no electrons. (static electricity)
false
Two neutral conducting pop cans are touching each other. A positively charged glass rod is brought near Can X as shown below. Which of the following occur as the glass rod approaches Can X? List all that apply. a. Electrons jump from the glass rod to can X. b. Electrons jump from the glass rod to can Y. c. Electrons jump from can X to the glass rod. d. Electrons jump from can Y to the glass rod. e. Protons jump from the glass rod to can X. f. Protons jump from can X to the glass rod. g. ... nonsense! None of these occur.
g Since contact is not made between the glass rod and Can X, there is no transfer of electrons between them. And of course, there is never a transfer of protons in electrostatic experiments. The glass rod simply induces the movement of electrons from Can Y to Can X, causing Can X to acquire a negative charge and Can Y to acquire a positive charge.
a skateboarder increases her speed from 2 m/sec to 4 m/sec. her momentum: (momentum and impulse)
increases
air bags help reduce injury in automobile accidents by: (momentum and impulse)
increasing the time that force is applied
the unit used to measure linear momentum is: (momentum and impulse)
kg*m/sec
if the speed of an object is doubled, which of the following must also double? (momentum and impulse)
momentum
metal sphere A has a mass of 5 kg, and a metal sphere B has a mass of 9 kilograms. the metal spheres are dropped from a 4 story window at the same time. as they reach the ground simultanously, metal sphere B has a larger: (momentum and impulse)
momentum
How much work is done by an applied force to lift a 15-Newton block 3.0 meters vertically at a constant speed? Work, Energy, and Power: Calculating the Amount of Work Done by Forces
o lift a 15-Newton block at constant speed, 15-N of force must be applied to it (Newton's laws). Thus, W = (15 N) * (3 m) * cos (0 degrees) = 45 Joules
a billiard ball collides with a second ball and stops. the total momentum of the billiard balls: (momentum and impulse)
remains the same
a force must be applied to stop a moving wagon. increasing the time interval over which the force is applied: (momentum and impulse)
requieres a smaller force
when comparing the momentum of two moving objects, which statement is ALWAYS correct? (momentum and impulse)
the object with higher velocity has greater momentum
an egg dropped on the sidewalk normally breaks, but an egg dropped on the gras might not break because: (momentum and impulse)
the time interval for stopping is greater when the egg hits the grass
jumping on a trampoline, jeffrey can easily soar 6 feet in the air. however, he can only jump 2 feet from the blacktop of the driveway basketball court because:(momentum and impulse)
the trampoline applies a bigger impulse
if the direction of the velocity is west, the direction of the momentum is: (momentum and impulse)
west
