Physics Exam 1
It was mentioned on page 59 that 65 mph equals 29.05 m/s. Show that this is indeed the case, using the fact that there are 1609 meters in a mile and 3600 seconds in an hour.
(65 mi/hr)x(1609 m/mi)x(1 hr/3600 s) = 29 m/s
(a) A car travels at an speed of 20 m/s. How long does it take for the car to travel 30 m? (b) A car travels at an speed of 20 feet per minute. How long does it take for the car to travel 30 feet?
(a) 1.5 s, (b) 1.5 minutes
An object starts at rest at 10 am and then speeds up, reaching a velocity of 20 m/s westward at 11 am. (a) What is ∆v? (b) Would your answer be any different if the object started with a velocity of 5 m/s westward at 10 am, instead of being at rest at 10 am? If so, in what way? If not, why not?
(a) 20 m/s westward, (b) The change in velocity would be 15 m/s westward, since it starts with a westward velocity and thus undergoes a smaller change.
Suppose an object is moving at 20 mph northward. (a) What is the value of ⃗v? (b) What is value of |⃗v|?
(a) 20 mph northward. (b) 20 mph.
At 0 s, the top car in Figure 7.2 is to the right of the other two, yet it ends up to the left because it is moving slower. (a) How can you tell that the middle car in Figure 7.2 is moving the fastest? (b) At 2 seconds, which car is in the lead? (c) When does the bottom car catch up to the top car? How do you know?
(a) Because the distance the car moves during each time increment is more than the others. (b) The middle car. (c) At 4 seconds. ⃗v = ∆⃗s ∆t
For each of the following situations, use the law of inertia (or force and motion) to determine whether the force on the object is zero or not: 1.) an object remains at rest 2.) an object starts to move northward 3.) An object is moving northwards, without speeding up or slowing down 4.) An object is moving northward and slowing down
The force of the object in zero in 1 & 3 because the objects motion (speed and direction) is not changing in those cases. The force on the object is not zero in 2 &4 because the motion is changing.
Suppose two cars, one moving at 50 mph and the other moving very slowly, are both put in neutral. The fast car slows down while the slow car continues to roll for a long time. Assuming both are on level ground, in which is drag more negligible?
When the car is moving slowly, since its motion isnt changing. The drag on the fast-moving car is responsible for slowing it down.
In which of the following situations is the object's CHANGE in velocity opposite the object's initial velocity?
When the object is slowing down
Suppose an object at rest has five forces acting on it. 3 downward, and 2 upward. IF the magnitudes are not all identical, is it possible for the force to be zero.
Yes it can if they are equal on both sides. 15 + 10 + 5 =30 27+3=30
If the object's acceleration has a constant non-zero value then could the object's velocity be zero?
Yes, but it will only be zero for an instant
The force and motion equation contains two quantities that involve directions: the net force and the change in velocity. Do the two directions have to be the same?
Yes, since they appear on opposite sides of the equals sign and there are no other quantities in the equation that include a direction. The equals sign means that the two sides are equal, not only in terms of the numerical value but also in terms of the units and the direction
Can an object be moving upward while accelerating downward? If not, why not? If so, provide a situation that illustrates this.
Yes. A baseball that has just been hit for a pop up and is still going up (but reducing its upward speed) is accelerating downward. A tennis ball that's been hit for a lob but is still on its way up is accelerating downward (i.e slowing down). An elevator moving upward but slowing down as it approaches the top floor is accelerating downward. And so on...
Is it possible for an object's velocity to be non-zero while its acceleration is zero? If so, describe a "real" situation involving something you do where that is the case. If not, explain why it is not possible.
Yes. Walking down the sidewalk at a constant speed and direction, or driving down a straight street are two such examples.
When entering Interstate 80, you essentially start from rest and reach the highway speed of 50 mph within the length of the on-ramp. Do you expect your acceleration during that period to be greater than the 0-60 acceleration of the 2015 Porsche 918 Spyder or less than that acceleration?
Your acceleration is less than the Spyder (assuming you are driving a typical car).
If an object's velocity is constant and non-zero (i.e., it is moving with a constant speed and direction) then what is the object's acceleration?
Zero (and remains zero during this time
As I ride an elevator down several floors, I find I move downward at a constant speed. What is the net force exerted on the elevator as it moves downward: zero or non-zero?
Zero net force --> no change in motion.
For a particular fan cart, we find that the speed of the fan cart does not remain constant but the direction does. What is the net force exerted on the fan cart as it moves: zero or non-zero?
Zero net force --> no change in motion.
An object is at rest. Suppose two forces are exerted upon it, equal in magnitude but opposite in direction. Does the object remain at rest? If so, why? If not, in which direction does the object start to move?
a) It stays at rest because the two forces are balanced, so the net (unbalanced) force is zero.
An object is set in motion. Once it is in motion, suppose two forces are exerted upon it. The two forces are equal in magnitude but opposite in direction. Does the object speed up or slow down? Why or why not?
a) It stays in constant motion because the two forces are balanced, so the net (unbalanced) force is zero.
The mass of a new pencil is about 5 g. Why is the mass given in units of grams instead of kilograms?
cause it involves less zeros. If we were to use the force and motion equation, we'd have to use kilograms (0.005kg)
Change in velocity is directly proportional to...
change in time
(a) Suppose an object takes 3 hours to move from a position 2 mi north of the PA/NY border to a position 5 mi north of the border. To find the average velocity, do you take the difference between the two and five or do you take the average of two and five? (b) Suppose an object takes 3 hours to accelerate uniformly from 2 mph northward to 5 mph northward. To find the average velocity, do you take the difference between the two values or do you take the average of two values?
(a) difference, (b) average
Suppose an object is moving at 10 m/s eastward. (a) What is the value of Suppose an object is moving at 10 m/s eastward. (a) What is the value of v? (b) What is the value of |⃗v|? Recall from section 4.1.1 that the absolute value symbols to indicate that we only want the magnitude
(a) ⃗v is the object's velocity, which is 10 m/s eastward. (b) |⃗v| is the object's speed, which is 10 m/s
Describe a real life situation if possible for each of the following cases if not explain why. 1.) the net force is exerted in the same direction as the motion of the object. 2.) the net force is exerted opposite the direction of objects motion
) If a net force is in the object's direction of motion it will speed up. An example would be pushing on the gas pedal of your car to pass a slower vehicle. b) If a net force is applied opposite to the object's direction of motion it will slow down. An example would be slamming on your brakes.
1 pound = ? kg
.45 kg
Suppose the net force on an object is zero. If it is moving during this time will it 1.)slow down 2.)speed up 3.)neither speed up or slow down?
Neither speed up or slow down
Change in velocity is directly proportional to
Net force
According to the law of force and motion, if no forces are acting upon an object, can the object be slowing down?
No
In Chapter 15 we'll define an object's momentum as the product of the object's mass and the object's velocity. Is this something that has been proven to be true? If not, could there be instances when the momentum is not equal to the product of the object's mass and its velocity?
No, It has not been proven to be true. We could choose to define momentum differently. However, as long as we have defined momentum to be the product of the objects's mass and its velocity it will always be the case
If an object has an acceleration of 5 m/s2 does that mean it is moving at a speed of 5 m/s? If not, does it mean it is moving faster than 5 m/s, slower than 5 m/s or is it not possible say?
No, it does not mean it is moving at a speed of 5 m/s. It is not possible say how fast the object is traveling. All we know is that it is accelerating.
Suppose a net force of 5 N northward acts for 3 seconds on an object of mass 2 kg. Can we use this information with the force and motion equation to determine how fast the object is moving at the end of the three seconds? If so, how fast is it moving? If not, why not?
No, the force and motion equation only tells us the change in the velocity. We don't even know whether it is speeding up or slowing down, because we aren't told which way the object is moving
Two forces are acting on an object. Is the object's velocity directly proportional to one of the two forces?
No, the object's change in velocity is directly proportional to the net force.
If I ride my bike up a side of a hill, slowing down as I do. During this time, is my inertia considered one of the forces acting on me?
No. An objects inertia is associated with the object itself, not its interaction with another object.
An object is being pushed across a frictionless floor with a force of 10 N to the right. No other forces are acting to speed it up or slow it down. If the object is moving toward the right, what is the object doing while the 10-N force is acting
Speeding up
The force and motion equation does not tell you how fast an object is moving or even the direction of motion, rather...
Tells how much the objects velocity is changing.
An object is observed to accelerate from 10 m/s southward to 25 m/s southward in a time of 5 seconds, What is the object's acceleration during this time? Show how you obtained your units of acceleration as well as the magnitude and direction.
The acceleration is Dv/ Dt... so the car accelerates by 15 m/s in 5s, so that gives an acceleration of (15 m/s)/5 s = 3 (m/s)/s Or 3 m/s2.
Newton's Second Law of Motion
The acceleration of an object depends on the mass of the object and the amount of force applied. If an objects motion is changing there must be a net (unbalanced) force exerted on that object)
(an object's) Displacement
is defined as change in position. displacement is a vector (has direction).
Suppose you are riding your bike up a hill at a constant speed. What can we say about the net force upon the bike?
it is zero
The magnitude of something is
just the number and unit portion
Force Imbalances
What remains after the forces cancel
Change in velocity=
Fnet/mass x Time
If an object is moving with a constant acceleration, does that mean it is moving with a constant velocity? If so, why? If not, why not?
NO! A constant acceleration is a constant change in velocity. Your velocity can't be both constant and changing can it? After all, constant means unchanging.
Suppose a force is equal to 2000 N northward. (a) What is Fnet ? (b) What is |Fnet |?
A.) 2000 N northward B.) 2000 N
When something moves at a constant speed
There is no force acting
An object is moving toward the right at a constant speed of 10 m/s. Which of the following is correct? A.The net force on the object is zero B.The net force on the object is toward the right C.The net force is unknown but there is a force on the object toward the right
A.The net force on the object is zero
My shoe is sliding on a frictionless surface. I then apply a force of 2 N westward for 0.8 s, during which time it accelerates with a magnitude of 5 m/s2 . What is the mass of my shoe? Can you say which way it is moving? If so, which? If not, why not?
Acceleration a = Dv/ Dt but is also Fnet/m. So if a= Fnet/m Then m= Fnet/a = (2 N)/ (5 m/s/s). We cannot say which way it's moving, because that would depend on its initial velocity which is not given.
A car is moving at 10 m/s northward. Fifteen seconds later, the car is moving at 5 m/s northward. (a) What is the direction of the car's acceleration? (b) In what direction is the net force acting on the object?
Both the acceleration and the net force are directed southward (opposite the direction of motion), since the object is slowing down.
An object is moving with a velocity of 20 miles per hour eastward. What is the object's displacement during two hours?
40 miles eastward
An object has three forces acting on it 5 N north word, 10 N north word and 20 N south word. What is the magnitude and direction of net force on the object?
5 N southward
Suppose a car's position changes by 10 meters in a time of 2 seconds. What is the car's velocity?
5 m/s
My 1000-kg car is rolling (without friction) with a velocity of 4 m/s westward. I find I can exert a force of 20 N eastward. How long does it take to slow the car down to 3 m/s westward?
50 s
A car maintains a constant speed of 12 m/s for 5 s. How far does the car travel during the 5 s?
60 m
Two forces are exerted on an object.One is 12 N eastward and the other is 20 N westward. What is the net force?
8 N westward
In which of the following situations does the object have constant speed? 1.) A rock at rest 2.) A ball that is rolling down a hill 3.) a bowling ball rolling across the floor without speeding or slowing down?
1 & 3
Starting at 10 am, the net force on an object is 10 N eastward. This lasts until 11 am. What is ∆t?
1 hour
A net force of 5 N toward the west is applied to an object for 3 seconds. If the object's initial velocity is 10 m/s toward the east and its final velocity is 5 m/s toward the west, what is the object's mass?
1 kg
Two students, Joe and Moe, are playing catch with a ball. At a particular moment, the ball is moving toward Joe. Joe then catches it and quickly returns it to moe. What is the direction of Joes force on the ball: 1.) As joe catches it in order to slow down 2.) as the ball is reversing direction 3.) As joe speeds up the ball during the act of throwing it toward Moe.
1.) Joe's force needs to be toward Moe, since the ball slowed down as it was moving toward joe (Opposite joes force) 2.) Joe's force continues to be toward Moe, since the ball is changing directions from moving toward Joe to moving Toward Moe. 3.) Joes force continues to be toward moe, since the ball is moving toward Moe and speeding up.
A rock is thrown up in the air. After being let go, it goes up in the air, slows down, and then comes back down. 1.) as the rock is going up and slowing down what is the direction of the net force acting on it? 2.) as the rock is reversing directions at the top what is the direction of the net force acting on it? 3.) as a rock is coming back down and speeding up what is the direction of the net force acting on it?
1.) The net force needs to be downward since the object slowed down when it was going up, opposite the net force. 2.) The net force needs to be downward since the object is changing directions from up to down 3.) The net force needs to be downward since the object is speeding up when it was going down, same direction as net force
Suppose a car takes 10 seconds to accelerate from 15 mph (6.7 m/s) to 60 mph (26.8 m/s). What is the car's acceleration during this time? a ->=(Δv ->)/Δt
2.01 m/s2
My 1000-kg car is rolling (without friction) with a velocity of 4 m/s westward. I find I can exert a force of 20 N eastward. How long does it take to stop the car?
200 s
Suppose the same force is exerted upon both a heavy bowling ball and a light Ping-pong ball for the same length of time. As a result, the bowling ball experiences a change in motion of 1 m/s. If the mass of the bowling ball is 2000 times greater than the mass of the Ping-pong ball, what is the Ping-pong ball's change in motion?
2000 m/s; it is greater because its mass is smaller
If the avg velocity is 22.5 m/s what is the initial and final?
20m/s east and 25m/s east
On average it travels 22.5m/s, after 20 seconds how much has it traveled?
22.5m/s x 20 s =450 m
m/s = n/kg
=(kg x m/s^2)/kg =m/s
Can there ever be a situation where an object slows down without any force being exerted upon it? If so, when? If not, why not?
An object cannot slow down with out the force of friction or air resistance. All of these are forces that act to slow down the object.
Newton's First Law
An object will not change its motion if the net (unbalanced force) is zero
Suppose the object experiences a CHANGE in velocity equal to 10 m/s toward the west. If its initial velocity is 5 m/s toward the east, what is its final velocity?
Answer is 5 m/s toward the west. In this case, the magnitude of the change is greater than the initial speed; the object slows down to a stop and turns around DURING the time interval
A car is driving eastward and slows down. While it is slowing down, in which direction is the car's acceleration? Explain your reasoning.
Any slowing down means the change in velocity is in the opposite direction of the motion, so the acceleration is in the opposite direction also (as is the net force... all three are always in the same direction, which in this case is westward!)
Suppose an object is accelerating at a rate of 2 m/s2 . If it starts with a speed of 10 m/s, how far does it travel in the next three seconds?
At an acceleration of 2 m/s2 , the object would speed up to 16 m/s by the end of the three seconds (a change of 6 m/s during the 3 seconds). The average of 10 and 16 is 13, so the average speed is 13 m/s. An object moving at that speed would travel 39 m in 3 s (multiply 13 by 3).
Vaverage->=Δs->/Δt
Average velocity is the change in position divided by the time it took for the position to change.
Suppose an object experiences a force for three seconds. During which of the following time intervals does the object experience a change in velocity? (a) Prior to when the force is present. (b) While the force is present. (c) After the force has ended and is no longer present. (d) During more than one of the above time intervals.
B
Why is the law of force and motion called a law?
Because it describes the relationship between force and motion.
An 2-kg object starts with a velocity of 3 m/s eastward and experiences a net force of 8 N westward for three seconds. What is the object's displacement during the three seconds?
Divide the net force by the mass to get an acceleration is 4 m/s2 , slowing down since the net force is opposite the motion. Multiply by 3 seconds to get a change in velocity equal to 12 m/s. This means the object slowed to a stop (change of 3 m/s) and then sped up to 9 m/s (for a total change of 12 m/s) in the opposite direction. The average of 3 m/s eastward and 9 m/s westward is 3 m/s westward (use a number line if needed). Over three seconds that corresponds to a displacement of 9 m westward.
In which of the following situations is the object's CHANGE in velocity opposite the net force acting on the object? A.When the object is at rest and staying at rest B.When the object is moving with a non-zero constant velocity C.When the object is speeding up D.When the object is slowing down E.None of the above; they are always in the same direction
E.None of the above; they are always in the same direction
For each of the following situations, determine whether the forces on the elevator balance or not (i.e., is the net force zero or not): (a) An elevator remains at rest on the ground floor. (b) The elevator starts moving upward. (c) As the elevator moves upward, without speeding up or slowing down. (d) As the elevator approaches the next floor, it slows to a stop
Forces are balanced, because an object at rest remains at rest only if the net force on it is zero. (b) Forces are not balanced, because if an object is speeding up (getting faster and faster), that means its motion is changing and thus requires a non-zero net force (upward, in the direction of motion). (c) Forces are balanced, because the object's motion (speed and direction) is not changing. (d) Forces are not balanced, because if an object is slowing down, that means its motion is changing and thus requires a non-zero net force (downward, opposite the direction of motion).
An object slows down at a rate of 5 m/s2 . At a certain time, it is measured to be moving at 20 m/s. How fast is it going 3 s later?
If it is slowing down at 5 m/s2 , that means the speed is decreasing by 5 m/s every second. Multiply by three seconds to get a change of 15 m/s. If it was initially moving at 20 m/s, then three seconds later it would be moving at 5 m/s.
An object speeds up at a rate of 5 m/s2 . At a certain time, it is measured to be moving at 20 m/s. How fast is it going 3 s later?
If it is speeding up at 5 m/s2 , that means the speed is increasing by 5 m/s every second. Multiply by three seconds to get a change of 15 m/s. If it was initially moving at 20 m/s, then three seconds later it would be moving at 35 m/s.
Suppose it takes 2 seconds to stop a particular object when applying a given net force. How long would it take to stop an object with triple the mass, assuming it was initially moving at the same speed and the same net force is applied?
If we go back to the version of the force-motion equation in the answer to 5.5: m = (Fnet/Dv) x Dt we see that the stopping time Dt is directly proportional to mass. (This makes sense because remember that the mass of an object determines the sluggishness of an object... so a more massive object will not only take longer to speed up, it will make it take longer to slow down or stop.) So 3 times the mass means 3 times longer stopping time or 6 seconds.
: Suppose it takes 2 seconds to stop a particular object when applying a given net force. How long would it take to stop an object with triple the mass, assuming it was initially moving at the same speed, if the magnitude of the net force is doubled?
If you rearrange the force & motion equation to solve for the time it takes to change an object's velocity from something to zero, the equation shows that the time is directly proportional to mass and inversely proportional to the net force. So if mass were tripled, the time it takes to slow the object down would also triple (because a more massive object has more inertia and is harder to change the motion). If the mass were doubled, the object would slow down sooner and the time necessary to stop it would only be 1/2. If both change together, the time to stop it would be 3/2 as long or 1.5 x as long.
Answer the following based on how we use the word acceleration in physics: (a) A car faces northward at a stoplight. When the traffic light turns green (or, for some people, when the light turns yellow), the car speeds up. Is this an acceleration? If so, in what direction? If not, why not? (b) A car is traveling northward when it encounters a stoplight. When the light turns yellow, cars are supposed to slow down. Is this an acceleration? If so, in what direction? If not, why not?
In physics, both are accelerations. The only difference is the direction of the acceleration. For (a) it is northward while for (b) it is southward.
A Child is playing with a yo-yo. The yo-yo is dropped and the child uses the string to pull the yo-yo back up. As the yoyo is reversing direction what is the direction of the force on the yo-yo?
In this case the net force is in opposite the initial direction since the yo-yo is initially going down word the net force on it must be upward
A 12-kg block is at rest on a horizontal frictionless surface. Three forces are acting on the block: (1) 10 N eastward, (2) 30 N westward and (3) 20 N eastward. Suddenly the 30 N force is switched from westward to eastward (still 30 N). This is then maintained for 2 s. How fast is the block moving at the end of the 2 s and in which direction?
Initially, the forces are balanced (30 N West is balanced by a total of 10N + 20 N East). When the westward force is switched to East, the net force is 60 N East. Using the law of force and motion the change in velocity = (60 N/12 kg) x 2s = 10 m/s. Since the block starts at rest, the final velocity is the same as the change in velocity.
If an objects acceleration is constant and non-zero then what is the objects velocity?
It has a value that changes during this time
: Is it possible for an object's velocity to be zero while its acceleration is non-zero? If so, describe a "real" situation where that is the case. If not, explain why not.
It is possible, but only for an instant, since if the acceleration is non-zero that means the velocity has to be changing. Some examples where it might be true for an instant include a car starting from rest or an object that is in the process of turning around (like a ball at the top of its motion, having been thrown straight up).
Suppose you are pushing a box along the floor. When you stop pushing, it slows down and stops. Why does it slow down.
It slows down because there is an imaginary force of friction along the floor. Friction causes the box to slow down.
Suppose an object starts with a velocity of 5 m/s eastward and accelerates at a rate of 2 m/s 2 westward for five seconds. When does the object's motion change directions? What is its velocity at the end of the five seconds?
It turns around at 2.5 seconds, ending up with a velocity of 5 m/s westward.
If you push a box across the floor will it speed up, slow down or move with a constant speed?
It will move with a constant speed if the net force is zero meaning the friction and the force I exert are equal. It will speed up if the force exert is more. It will slow down if the force I exert is less.
Derive equation 6.2 (⃗a = F⃗ net/m) from our previous version of the law of force and motion (∆⃗v = F⃗ net∆t/m).
Just divide both sides of the equation by Dt and the left side becomes Dv /Dt, which is acceleration.
Show algebraically how the units of N·s/kg are equivalent to m/s, given that one newton is equivalent to one kg·m/s2 .
N s/kg = (kg m/s2) x (s/kg) the s cancels out one of the s2 and the kg on top cancels the kg on the bottom. What is left is m/s.
Suppose the net force is 5 N toward the west. In what direction is the velocity?
N/A the net force only tells us how the velocity is changing
If the force is given in newtons, the time is in hours and the mass is in pounds, and we plugged the values into the force and motion equation, would the result give us the change in motion in meters per second (without any further unit conversions)? Why or why not?
No. The result is in meters per second only if you use SI units throughout, which means newtons for force, seconds for time, and kilograms for mass
The law of gravity is often called Newton's law of gravitation. Does that mean Newton was the first to prove this law?
No. While the law of gravity is well-supported by observations and tests, it isn't necessarily true or proven to be true. Rather, it is called a law because it is a general relationship that can be used to make predictions. Newtons name is used. because he was among the first to officially describe it.
As I ride my bicycle up a hill, I find I slow down. What is the net force exerted on the bicycle as it moves: zero or non-zero?
Non-zero net force --> change in motion.
For a particular toy car, we find that the speed (and direction) of the toy car remains constant. What is the net force exerted on the toy car as it moves: zero or non-zero?
Non-zero net force --> change in motion.
What are representations of opposite directions?
North & South Positive & Negative
Consider an object that is initially moving 3 m/s southward and then turns around and moves 2 m/s northward. In what direction is the object's acceleration while it is turning around (from southward to northward)?
Northward.
I have two identical objects and the same non-zero force is exerted on each but for different amounts of time. Object A experiences the force for 5 seconds. Object B experiences the force for 10 seconds. Which object experiences the greater change in velocity?
Object B. The law of force and motion says that the longer the force is applied, the greater the object's change in velocity.
State the law of force and motion in words:
Objects motion changes when a force is acting upon it. The forces act to change the motion of the object, (and conversely, that the motion doesn't change where there are no forces acting on the object.)
For objects at rest or moving slowly through the air, is it valid to ignore the force due to air on objects?
Probably, because the drag is likely to be insignificant unless the object is moving quickly through the air or the air is moving very quickly past the object.
Δa= Δv->/Δt
Rate of change of velocity
What net force is required to slow down a 5-kg object from 4 m/s westward to 1 m/s westward in 1.5 seconds?
Since everything is in SI units, the units will work out and so we can focus only on the numerical values. In this case, the object slows down by 3 m/s, so that is what we use in the equation: 3 = F⃗ net x 1.5 / 5
My shoe is sliding on a frictionless surface with a velocity of 2 m/s westward. I then apply a force of 2 N westward for 0.8 s. At the end of the 0.8 s, my shoe has a velocity of 6 m/s westward. What is the mass of my shoe?
So, we need to solve the equation of force and motion for mass. After a little algebra we find that: m = (Fnet/Dv) x Dt . Since we have a change of velocity of 4 m/s, when we plug in the numbers we get m = (2 N/4 m/s) x 0.8 s = 0.4 kg.
A 2-kg box is on a horizontal, frictionless surface with an initial velocity of 4 m/s leftward. A constant net force of 30 N rightward is applied on the box for 0.2 seconds. How far does the box travel during the 0.2 seconds?
The box is slowing down, since the force is rightward while the box is moving leftward. The acceleration is 15 m/s2 (use force and motion equation; divide net force by mass). During the 0.2 s, that means the object slows down by 3 m/s (multiply acceleration by the time). Since it started at 4 m/s, that means it is going 1 m/s by the end of the 0.2 s. Since the net force is constant, so is the acceleration and so the average speed is midway between 1 m/s and 4 m/s. That would give an average speed of 2.5 m/s. At that speed, for 0.2 s, an object would travel 0.5 m (multiply average speed by the time).
Suppose an object's motion changes by 5 m/s when the object experiences a net force of magnitude 10 N for 3 seconds. What would be the change of motion for the same object if the magnitude of the net force is half as much (5 N) for 3 seconds? Why don't you need to know the object's mass for this problem?
The change in motion is 2.5 m/s, half of what it was with the 10 N net force, since the magnitude of the net force is half of what it was when the change in velocity was 5 m/s. We don't need to know the mass since it is the same object, and thus the same mass in each case.
In the previous checkpoint, a net force of 5 N northward acts for 3 seconds on an object of mass 2 kg. Determine the direction of the object's change in velocity during the 3 seconds for each of the following cases. (a) If the object is initially moving at 5 m/s northward. (b) If the object is initially moving at 10 m/s southward. (c) If the object is initially moving at 5 m/s southward.
The change in velocity is northward in all three cases, the same direction as the net force. However, this corresponds to a speeding up in (a), a slowing down in (b) and change in directions in (c).
Suppose an object experiences a change in velocity of 7.5 m/s northward when a given net force acts upon it for a given amount of time. What will be the change in velocity if the object is replaced with one that is three times the mass and we triple the magnitude of the net force?
The change in velocity would be the same as before, 7.5 m/s northward
Suppose a net force of 5 N northward acts for 3 seconds on an object of mass 2 kg. Determine the object's velocity at the end of the 3 seconds for each of the following cases. (a) If the object is initially moving at 5 m/s northward. (b) If the object is initially moving at 10 m/s southward. (c) If the object is initially moving at 5 m/s southward.
The change is 7.5 m/s in all three cases but the final velocity is 12.5 m/s northward in (a), 2.5 m/s southward in (b) and 2.5 m/s northward in (c).
An eight-fluid-ounce bottle or glass of water is about 240 ml (i.e., 240 milliliters). There are 1000 ml in a liter. What is the mass of the 240 ml of water?
The density of water is 1 g/ml or .001 kg/ml. So a 240 ml glass of water has a mass of 240 g or 0.240 kg
At the moment an object is moving northward, a northward directed force(pushing the object north) is being exerted on it. While the force is acting, it the object speeding up, slowing down, or neither?
The force is in the same direction of motion. The object will be speeding up.
Suppose an object is slowing down. Which would have the largest magnitude: the initial velocity, the final velocity or the average velocity? Give an example in support of your answer
The initial velocity. Consider, for example, an object that starts with a speed of 10 m/s and slows down to 5 m/s. The average must be somewhere between 10 m/s and 5 m/s
As I ride my bicycle down a hill I apply my brakes in such a way that I move with the constant speed down the hill. During this time, when my speed and direction is constant, is the net force exerted upon the bicycle zero? Explain how the law of force and motion can be used to answer this question.
The law of Force and motion says when the net force acting on the object is zero the forces are balanced and the motion does not change the object is an equilibrium.
A heavy bowling ball and a light Ping-pong ball are both at moving at 10 m/s. I exert a 10-N force on each object, opposite their motion. Which object stops in less time? Why?
The light Ping-pong ball. Since ∆t and m are both on the right side of the force and motion equation, with ∆t in the numerator and m in the denominator, they must be directly proportional (i.e., their quotient must remain the same, since all the other quantities stay the same). That means that the more massive object (the bowling ball) will take longer to stop (since the net force on each is the same and both were initially moving with the same speed).
Suppose you are seated on a plane that is cruising at a steady speed of 250 m/s (about 560 mph) in a constant direction. According to the law of force and motion, is the net force on you zero? Why or why not?
The net force on me on an airplane at cruising altitude in constant uniform motion is zero. Again, this is because there is no change in motion. (Motion does not require a net force, only a change in motion does!)
In Acceleration if the velocity is zero...
The object is stationary or at rest, or stays at rest or- object is moving w/ a constant velocity
To start an object moving, there has to be a force imbalance. What happens if, once moving, the forces change such that they then become balanced?
The object will maintain the same speed it had at the moment the forces became balanced
I have two objects of identical mass. Object A experiences a net force of 10 N westward while object B experiences a net force of 20 N westward. During the same period of time, which object experiences the greater change in velocity?
The object with the greater net force (B) experiences the greater change in velocity
When something speeds up.....
There is a force acting in the same direction as motion
Suppose an object is moving toward the right while the net force exerted on it is toward the left. What is the object's acceleration?
To the left, as is the net force
Suppose the net force on a 2-kg object is 5 N toward the west for 3 s. If the object's initial velocity is 10 m/s toward the east, in what direction is the final velocity? Hint:Δv ⃗ =F ⃗_net/m Δt
Toward the east The change in velocity is 7.5 m/s to the west; which means the object slows by 7.5 m/s but is still moving 2.5 m/s toward the east
An object is moving toward the right when a net force is exerted on it toward the left, making the object slow down, stop for an instant and then move back the other way. At the moment the object is stopped, the net force is:
Toward the left
Suppose the net force is 5 N toward the west. In what direction is the change in velocity?
Toward the west
Suppose a 4-kg object is experiencing a net force of 20 N northward. If it starts with a velocity of 10 m/s northward, how far does it travel in two seconds?
Use the force and motion equation to get an acceleration of 5 m/s2 (northward), which means it is speeding up (since it is being pushed in the direction of motion). At an acceleration of 5 m/s2 , the object would speed up to 20 m/s by the end of the two seconds (a change of 10 m/s during the two seconds). The average of 10 and 20 is 15, so the average speed is 15 m/s. An object moving at that speed would travel 30 m in 2 s (multiply 130 by 2).
For each of the following situations, identify whether the net force exerted on the object must be zero according to the law of force and motion. If it is explain why, if it isn't identified the direction of the net force. (a) A sled, initially at rest, is given a push to get it moving. (b) By pushing a book horizontally against the wall, one can keep it at rest (without it falling or sliding down the wall). (c) A person pushes against a large box that sits on the floor but, due to friction, the box stays put. (d) An elevator that starts moving upward just after the doors close. (e) An elevator that is sitting at a floor with the doors open. (f) You, standing in an elevator, waiting for the doors to close. (g) A person pushes a large box across the floor such that it moves with a constant speed along a straight line.
a) Net force --> change in motion. b) No net force --> no change in motion. c) No net force --> no change in motion. d) Net force --> change in motion. e) No net force --> no change in motion. f) No net force --> no change in motion. g) No net force --> no change in motion. If you haven't noticed, the key word is change in motion... not motion.
Suppose an object experiences a change in velocity of 7.5 m/s northward when a given net force acts upon it for a given amount of time. Determine the change in velocity for the following cases. (a) If the magnitude of the net force is tripled. (b) If the object is replaced with one that is three times the mass.
a) Since Dv is directly proportional to Fnet, then if the net force is tripled, the Dv will also triple to 22.5 m/s Northward. b) Since Dv is inversely proportion to mass, if the mass is tripled, the Dv drops to 1/3 rd or 2.5 m/s Northward.
I observe a 10-kg object decelerate from 6 m/s eastward to 4 m/s eastward in 1 second. (a) Given that the object had slowed by 2 m/s, what What is the object's instantaneous velocity at the initial time? (b) What is the object's instantaneous velocity at the final time? (c) What is the magnitude of the object's change in velocity? (d) Using the answer in (c) and the force and motion equation, what is the magnitude of the net force acting on the object? (e) Is the object speeding up or slowing down? (f) Using the answer in (e), what is the direction of the net force acting on the object?
a) The question says that the initial instantaneous velocity is 6 m/s East. b) Again, as stated in the question the final instantaneous velocity is 4 m/s East. c) The magnitude of the Dv is a change of speed of 2 m/s. d) Fnet =( m x Dv) /Dt = (10 kg x 2 m/s West) / 1 s = 20 N. e) It's slowing down. f) Since the initial velocity is eastward and the object is slowing down, the net force is toward the west.
For each of the following, describe a situation, if possible, where an object undergoes a change in velocity that has the property given. If it is not possible, explain why. (a) An acceleration in the same direction as the net force on the object. (b) An acceleration opposite the direction of the net force on the object. (c) An acceleration in the same direction as the object's velocity. (d) An acceleration opposite the direction of the object's velocity
a) acceleration is ALWAYS in the same direction as the net force... so take your pick. b) They are never opposite, so there is no situation that can be given for b. c) When you are speeding up to pass a truck on a highway. d) When you put on the brakes to slow down at a red light.
non-zero net force
an unbalanced force that results in the motion of an object or a change in speed or direction of a moving object. some value that is not changing
An object reverses direction when....
if the force starts out opposite the direction of motion, momentarily stops, and then becomes the same direction
Which of the following ways are equivalent to the force and motion equation 4.1, in the sense that it can be obtained from equation 4.1 via algebraic manipulation? For each, explain why it is equivalent or not. (a) ∆t = (Fnet/m)∆v (b) ∆v/∆t = Fnet/m (c) Fnet = m∆v/∆t
b/c
Velocity (v->)
is both speed and direction of motion
When the net force acting on the object is zero, the forces are balanced, the object is in....
equilibrium
If velocity is not constant
have to determine the average velocity
Speed is...
how fast an object is moving
Change in velocity is inversely proportional to
mass
How do we distinguish between the unit abbreviations for meter, minute and mile? Check appendix C.
meter= m Minute = min mile= mi
An object is being pushed across a floor with a force of 10 N to the right. This force is countered by a 10 N friction force acting to the left. If the object is moving toward the right, what is the object doing while both forces are acting
moving with a constant speed (& direction)
Δs->
position
An object is being pushed across a floor with a force of 10 N to the right. This force is countered by a 20 N friction force acting to the left. If the object is moving toward the right, what is the object doing while both forces are acting
slowing down
If the velocity is changing uniformly,
the average velocity is the midrange value between the initial +final values
When an object slows down
theres a force opposite the direction of motion
Round trip=
velocity is zero
To make an object speed up...
you exert a force the same direction as motion, and vice versa to slow down
