physics

e) For any given motion, it is possible that an object could move very fast yet have an abnormally small velocity.

.e. TRUE - An Indy Race car driver is a good example of this. Such a driver is obviously moving very fast but by the end of the race the average velocity is essentially 0 m/s.

a. A cable is attached to a bucket and the force of tension is used to pull the bucket out of a well

A. Positive Work

11. A fullback is running down the football field in a straight line. He starts at the 0-yard line at 0 seconds. At 1 second, he is on the 10-yard line; at 2 seconds, he is on the 20-yard line; at 3 seconds, he is on the 30-yard line; and at 4 seconds, he is on the 40-yard line. What is the player's acceleration?

Answer: 0 m/s/s The fullback is moving 10 yards every second. He has a constant speed. He also is running in a straight line, so he is not changing direction. Thus, his acceleration is 0 m/s/s. Only objects with changing velocity have a nonzero acceleration.

18. Which one of the following statements is NOT true of a free-falling object? An object in a state of free fall ____. 1. falls with a constant speed of -10 m/s. 2. falls with a acceleration of -10 m/s/s. 3. falls under the sole influence of gravity. 4. falls with downward acceleration which has a constant magnitude.

Answer: A A free-falling object is an object upon which the only force is gravity. As it falls, it accelerates at a rate of approx. 10 m/s/s. This acceleration value is constant during the entire trajectory of the motion. Since this is the case, the speed can not be constant.

21. An object is dropped from rest. As the object freely falls, its ____. a. speed increases b. acceleration increases c. both of these d. none of these

Answer: A As an object falls, it accelerates; this means that the speed will be changing. While falling, the speed increases by 10 m/s every second. The acceleration is a constant value of 10 m/s/s; thus, choice b should not be chosen.

13. If an object is moving eastward and slowing down, then the direction of its velocity is ____. a. eastward b. westward c. neither d. not enough info to tell

Answer: A The direction of the velocity vector is always in the same direction as the direction which the object moves.

14. If an object is moving eastward and slowing down, then the direction of its acceleration is ____. a. eastward b. westward c. neither d. not enough info to tell

Answer: B If an object is slowing down, then the direction of the acceleration vector is in the opposite direction as the direction which the object moves. (If the object were speeding up, the acceleration would be eastward.)

22. A speedometer is placed upon a free-falling object in order to measure its instantaneous speed during the course of its fall. Its speed reading (neglecting air resistance) would increase each second by ____. a. about 5 m/s b. about 10 m/s c. about 15 m/s d. a variable amount e. depends on its initial speed.

Answer: B The acceleration of gravity is approximately 10 m/s/s. Acceleration represents the rate at which the velocity changes - in this case, the velocity changes by 10 m/s every second. So the speed will increase by the amount of 10 m/s every second.

19. When a rock is dropped, it will accelerate downward at a rate of 9.8 m/s 2 . If the same rock is thrown downward (instead of being dropped from rest), it acceleration will be ____. (Ignore air resistance effects.) a. less than 9.8 m/s 2 b. 9.8 m/s 2 c. more than 9.8 m/s 2

Answer: B Whether rising or falling, if the sole force acting upon the object is gravity, then the acceleration is 9.8 m/s/s (often approximated as 10 m/s/s).

16. Which one of the following quantities is NOT a speed? a. 10 mi/hr b. 10 mi/hr/sec c. 35 m/s d. 20 m/s

Answer: B You can often tell a quantity by its units. 10 mi/hr/sec is an acceleration since there are two time units involved. In fact, the units are velocity change units (mi/hr) per time units (seconds). The quantity speed has units of distance/time.

17. A baseball pitcher delivers a fast ball. During the throw, the speed of the ball increases from 0 to 30.0 m/s over a time of 0.100 seconds. The average acceleration of the baseball is ____ m/s 2 . a. 3.00 b. 30.0 c. 300. d. 3000 e. none of these

Answer: C Acceleration is velocity change over time. In this problem, the velocity change is +30.0 m/s and the time is 0.100 s. Thus,

9. Which one of the following is NOT consistent with a car which is accelerating? a) A car is moving with an increasing speed. b) A car is moving with a decreasing speed. c) A car is moving with a constant, high speed. d) A car is changing direction.

Answer: C An accelerating object must be changing its velocity by either slowing down, speeding up or changing direction. Moving fast merely means that the velocity is high; it says nothing about the acceleration.

23. Ten seconds after being dropped from rest, a free-falling object will be moving with a speed of ____. a. about 10 m/s. b. about 50 m/s. c. about 100 m/s. d. more than 100 m/s.

Answer: C Since the speed of a free-falling object increases by 10 m/s every second, the speed after ten of these seconds will be 100 m/s. You could use the kinematic equation v f = v i + a*t where v i =0 m/s and a = -10 m/s/s and t=10

10. A fullback is running down the football field in a straight line. He starts at the 0-yard line at 0 seconds. At 1 second, he is on the 10-yard line; at 2 seconds, he is on the 20-yard line; at 3 seconds, he is on the 30-yard line; and at 4 seconds, he is on the 40-yard line. This is evidence that a) he is accelerating b) he is covering a greater distance in each consecutive second. c) he is moving with a constant speed (on average).

Answer: C The fullback is moving 10 yards every second. He has a constant speed and thus covers the same distance (10 yd) in each consecutive second. He is not accelerating.

7. If an object has an acceleration of 0 m/s 2 , then one can be sure that the object is not ____. a. moving b. changing position c. changing velocity

Answer: C The object could be moving and could be at rest; however, whether moving or not, it must not have a changing velocity.

18. On takeoff, a rocket accelerates from rest at a rate of 50.0 m/s 2 for exactly 1 minute. The rocket's speed after this minute of steady acceleration will be ____ m/s. a. 50.0 b. 500. c. 3.00 x 10 3 d. 3.60 x 10 3 e. none of these

Answer: C Use the equation v f = v i + a*t v f = 0 + (50.0 m/s/s)*(60.0 s) = 3.00 x 10 3 m/s (Note that the unit on time must be the same as the time units for which the acceleration is given.)

15. Which one of the following quantities is NOT a vector? a. 10 mi/hr, east b. 10 mi/hr/sec, west c. 35 m/s, north d. 20 m/s

Answer: D A vector has both magnitude and direction. Only choice d does not show a direction; it must be a scalar.

8. Car A and Car B are driving in the same direction. If car A passes car B, then car A must be ____. a) accelerating. b) accelerating at a greater rate than car B. c) moving faster than car B and accelerating more than car B. d) moving faster than car B, but not necessarily accelerating.

Answer: D All that is necessary is that car A has a greater speed (is moving faster). If so, it will eventually catch up and pass car B. Acceleration is not necessary to overcome car B; a car going 60 mi/hr at a constant speed will eventually pass a car going 50 mi/hr at a constant speed. Surely you have witnessed that while driving down a local highway.

12. Olympic gold medalist Michael Johnson runs one lap around a circular track - 400 meters - in 38 seconds. What is his displacement? ___________ What is his average velocity? ___________

Answer: d = 0 m and v = 0 m/s Michael finishes where he started, so he is not "out of place." His displacement is 0 meters. Since average velocity is displacement over time, his average velocity is also 0 m/s.

A Watt is the standard metric unit of work.

FALSE - Watt is the standard metric unit of power; Joule is the standard metric unit of energy

a) Distance is a vector quantity and displacement is a scalar quantity.

a. FALSE - Distance is the scalar and displacement is the vector. Know this one!

Kinetic energy is the form of mechanical energy which depends upon the position of an object.

a. FALSE - Kinetic energy depends upon the speed of the object; potential energy depends upon the position of the object.

a) Position-time graphs cannot be used to determine if an object is changing its velocity.

a. FALSE - Position-time graphs represent change in velocity by the graph changing slope.

Moving objects cannot have potential energy.

a. FALSE - Potential energy has nothing to do with speed; an object could be moving at an elevated position. It is this elevation above zero level which gives an object potential energy.

a) The slope on a velocity-time graph is representative of the acceleration of the object.

a. TRUE - Now this is important! It is the beginning point of much of our discussion of velocity-time graphs. The slope equals the acceleration.

Work is a form of energy.

a. TRUE - Work is a form of energy, and in fact it has units of energy

a) Acceleration is a vector quantity (has a direction).

a. TRUE - Yes it is. Acceleration is direction-conscious.

a) Velocity is a vector quantity (has direction) and speed is a scalar quantity (has no direction).

a. TRUE - Yes! Speed is a scalar and velocity is the vector. Know this one!

a. An object that is free-falling is acted upon by the force of gravity alone

a. TRUE - Yes! This is the definition of free fall.

b. Rusty Nales uses a hammer to exert an applied force upon a stubborn nail to drive it into the wall.

a. positive work

g. In a physics lab, an applied force is exerted parallel to a plane inclined at 30-degrees in order to displace a cart up the incline.

a. positive work

h. A pendulum bob swings from its highest position to its lowest position under the influence of the force of gravity.

a.positive work

b) Accelerating objects MUST be changing their speed.

b. FALSE - Accelerating objects could be changing their speed; but it is also possible that an accelerating object is only changing its direction while maintaining a constant speed. The race car drivers at Indy might fit into this category (at least for certain periods of the race).

b. A falling skydiver which has reached terminal velocity is considered to be in a state of free fall.

b. FALSE - Skydivers which are falling at terminal velocity are acted upon by large amounts of air resistance. They are experiencing more forces than the force of gravity. As such, they are NOT free-falling.

b) Both speed and velocity refer to how fast an object is moving.

b. FALSE - Speed refers to how fast an object is moving; but velocity refers to the rate at which one's motion puts an object away from its original position. A person can move very fast (and thus have a large speed); but if every other step leads in opposite directions, then that person would not have a large velocity.

b) The slope on a position-time graph is representative of the acceleration of the object.

b. FALSE - The slope of a position-time graph is the velocity of the object. Some things in this unit are critical things to remember and internalize; this is one of them.

b) A person makes a round-trip journey, finishing where she started. The displacement for the trip is 0 and the distance is some nonzero value.

b. TRUE - Displacement is the change in position of an object. An object which finishes where it started is not displaced; it is at the same place as it started and as such has a zero displacement. On the other hand, the distance is the amount of ground which is covered. And if it was truly a journey, then there is definitely a distance.

If an object is at rest, then it does not have any kinetic energy.

b. TRUE - Kinetic energy depends upon speed. If there is no speed (the object is at rest), then there is no kinetic energy.

b) The area on a velocity -time graph is representative of the change in position of the object.

b. TRUE - This is equally important. The area is the displacement.

. Near the end of the Shockwave ride, a braking system exerts an applied force upon the coaster car to bring it to a stop.

b. negative work

f. In baseball, the catcher exerts an abrupt applied force upon the ball to stop it in the catcher's mitt.

b. negative work

d. The force of friction acts upon a baseball player as he slides into third base.

b. negitive work

c) An accelerated object's motion will be represented by a curved line on a velocity-time graph.

c. FALSE - An object which has an acceleration will be represented by an line that has a slope. It may or may not curve, but it must have a slope other than zero.

c. A ball is thrown upwards and is rising towards its peak. As it rises upwards, it is NOT considered to be in a state of free fall.

c. FALSE - Any object - whether rising, falling or moving horizontally and vertically simultaneously - can be in a state of free fall if the only force acting upon it is the force of gravity. Such objects are known as projectiles and often begin their motion while rising upwards.

If an object is on the ground, then it does not have any kinetic energy.

c. FALSE - If an object is on the ground, then it does not have potential energy (relative to the ground).

c) Person X moves from location A to location B in 5 seconds. Person Y moves between the same two locations in 10 seconds. Person Y is moving with twice the speed as person X.

c. FALSE - Person Y has one-half the speed of Person X. If person Y requires twice the time to do the same distance, then person Y is moving half as fast.

c) A person starts at position A and finishes at position B. The distance for the trip is the length of the segment measured from A to B.

c. FALSE - This would only be the case if the person walk along a beeline path from A to B. But if the person makes a turn and veers left, then right and then ..., then the person has a distance which is greater than the length of the path from A to B. Distance refers to the amount of ground which is coverd

Work is a time-based quantity; it is dependent upon how fast a force displaces an object.

c. FALSE - Work is not dependent on how rapidly the force displaces an object; power is time-based and calculated by force multiplied by speed.

c) A straight, diagonal line on a position-time graph is representative of an object with a constant velocity.

c. TRUE - A straight diagonal line is a line of constant slope. And if the slope is constant, then so is the velocity.

c) Accelerating objects MUST be changing their velocity.

c. TRUE - Accelerating object MUST be changing their velocity -either the magnitude or the direction of the velocity.

e. A busy spider hangs motionless from a silk thread, supported by the tension in the thread.

c. no work

The kinetic energy of an object is dependent upon the weight and the speed of an object.

d. FALSE (sort of) - Kinetic energy depends upon mass and speed. Two objects of the same mass could have different weights if in a different gravitational field; so it is not appropriate to say that kinetic energy depends upon weight.

d) Objects with positive acceleration will be represented by upwardly-curved lines on a velocity- time graph.

d. FALSE - An object with positive acceleration will have an positive or upward slope on a v-t graph. It does not have to be a curved line. A curved line indicates an object that is accelerating at a changing rate of acceleration.

d) If an object is at rest, then the position-time graph will be a horizontal line located on the x- axis.

d. FALSE - Not necessarily true. If the object is at rest, then the line on a p-t graph will indeed be horizontal. However, it will not necessarily be located upon the time axis.

Superman applies a force on a truck to hold it at rest and prevent it from moving down a hill. This is an example of work being done.

d. FALSE - Since Superman does not cause a displacement, no work is done; he is merely holding the car to prevent its descent down the hill.

d) Acceleration units include the following; m/s 2 , mi/hr/sec, cm/s 2 , km/hr/m.

d. FALSE - The first three sets of units are acceleration units - they include a velocity unit divided by a time unit. The last set of units is a velocity unit divided by a length unit. This is definitely NOT an acceleration.

d) If a person walks in a straight line and never changes direction, then the distance and the

d. TRUE - If a person never changes direction and maintains the same heading away from the initial position, then every step contributes to a change in position in the same original direction. A 1 m step will increase the displacement (read as out of place-ness) by 1 meter and contribute one more meter to the total distance which is walked.

d. An object in free fall experiences an acceleration which is independent of the mass of the object.

d. TRUE - The unique feature of free-falling objects is that the mass of the object does not effect the trajectory characteristics. The acceleration, velocity, displacement, etc. is independent of the mass of the object.

d) The velocity of an object refers to the rate at which the object's position changes.

d. TRUE - Yes! That is exactly the definition of velocity - the rate at which velocity changes.

e) The phrase "20 mi, northwest" likely describes the distance for a motion.

e. FALSE - Distance is a scalar and is ignorant of direction. The "northwest" on this quantity would lead one to believe that this is a displacement (a vector quantity) rather than a distance.

Faster moving objects always have a greater kinetic energy.

e. FALSE - Faster moving objects would have more kinetic energy than other objects of the same mass. However, another object could have less speed and make up for this lack of speed in terms of a greater mass.

e. A ball is thrown upwards, rises to its peak and eventually falls back to the original height. As the ball rises, its acceleration is upwards; as it falls, its acceleration is downwards.

e. FALSE - The acceleration of all free-falling objects is directed downwards. A rising object slows down due to the downward gravity force. An upward-moving object which is slowing down is said to have a downwards acceleration.

An upward force is applied to a hold a bucket as it is carried 20 m horizontally across the yard at constant speed. This is an example of work being done.

e. FALSE - The upward force does not cause the horizontal displacement so this is a NON-example of work.

e) Accelerated objects are represented on position-time graphs by lines with changing slope.

e. TRUE - Accelerating objects (if the acceleration is attributable to a speed change) are represented by lines with changing slope - i.e., curved lines.

e) If an object is at rest, then the velocity-time graph will be a line with zero slope.

e. TRUE - An object that is at rest has a 0 velocity and maintains that zero velocity. The permanence of its velocity (not the fact that it is zero) gives the object a zero acceleration. and as such, the line on a v-t graph would have a slope of 0 (i.e., be horizontal).

e) The direction of the acceleration vector is dependent upon two factors: the direction the object is moving and whether the object is speeding up or slowing down.

e. TRUE - This is the case and something important to remember. Consider its application in the last three parts of this question.

f) A line with zero slope on a velocity-time graph will be representative of an object which is at rest.

f. FALSE - A line with zero slope is representative of an object with an acceleration of 0. It could be at rest or it could be moving at a constant velocity.

More massive objects always have a greater kinetic energy.

f. FALSE - More massive objects would have more kinetic energy than other objects with the same speed. However, another object could have less mass and make up for this lack of mass in terms of a greater speed.

f) The phrase "30 mi/hr, west" likely refers to a scalar quantity.

f. FALSE - The presence of the direction "west" in this expression rules it out as a speed expression. Speed is a scalar quantity and direction is not a part of it.

f) An object which is slowing down has an acceleration.

f. TRUE - Accelerating objects are either slowing down, speeding up or changing directions.

f. A ball is thrown upwards, rises to its peak and eventually falls back to the original height. The speed at which it is launched equals the speed at which it lands. (Assume negligible air resistance.)

f. TRUE - If the object is truly in free-fall, then the speed of the object will be the same at all heights - whether its on the upward portion of its trajectory or the downwards portion of its trajectory. For more information, see the Projectiles page at The Physics Classroom.

f) An object with a positive velocity will be represented on a position-time graph by a line with a positive slope.

f. TRUE - Since slope on a p-t graph represents the velocity, a positive slope will represent a positive velocity.

f) The phrase "20 m, west" likely describes the displacement for a motion.

f. TRUE - The unit is an appropriate displacement unit (length units) and the direction is stated. Since there is both magnitude and direction expressed, one would believe that this is likely a displacement.

A force is applied by a chain to a roller coaster car to carry it up the hill of the first drop of the Shockwave ride. This is an example of work being done.

f. TRUE - There is a component of force in the direction of displacement and so this is an example of work.

g) A line with a negative slope on a velocity-time graph is representative of an object with negative velocity.

g. FALSE - A negative slope indicates a negative acceleration. The object could be moving in the positive direction and slowing down (a negative acceleration).

g) The average velocity of an object on a round-trip journey would be 0.

g. TRUE - For a round trip journey, there is no ultimate change in position. As such, the average velocity is 0 m/t seconds. Regardless of the time, the average velocity will be 0 m/s.

Kinetic energy is a scalar quantity.

g. TRUE - Kinetic energy does not have a direction associated with it; it is a scalar quantity.

g) An object with a negative velocity will be represented on a position-time graph by a line with a negative slope.

g. TRUE - Since slope on a p-t graph represents the velocity, a negative slope will represent a negative velocity.

g. A very massive object will free fall at the same rate of acceleration as a less massive object.

g. TRUE - The acceleration of free-falling objects (referred to as the acceleration of gravity) is independent of mass. On Earth, the value is 9.8 m/s/s (the direction is down). All objects - very massive and less massive - experience this acceleration value.

The force of friction acts upon a softball player as she makes a headfirst dive into third base. This is an example of work being done

g. TRUE - There is a force and a displacement; the force acts in the opposite direction as the displacement and so this force does negative work.

g) An object which is moving at constant speed in a circle has an acceleration.

g. TRUE - To move in a circle is to change one's direction. As such, there is a change in the velocity (not magnitude, but the direction part); this constitutes an acceleration.

h) If an object changes its direction, then the line on the velocity-time graph will have a changing slope.

h. FALSE - An object which changes its direction will be represented by a line on a v-t graph that crosses over the time-axis from the + velocity region into the - velocity region.

h) The direction of the velocity vector is dependent upon two factors: the direction the object is moving and whether the object is speeding up or slowing down.

h. FALSE - The direction of the velocity vector depends only upon the direction that the object is moving. A westward moving object has a westward velocity.

An object is moving to the right. A force acts leftward upon it. This force is doing negative work.

h. TRUE - A force which acts in a direction opposite the motion of an object will do negative work.

An object has a kinetic energy of 40 J. If its mass were twice as much, then its kinetic energy would be 80 J.

h. TRUE - Kinetic energy is directly related to the mass of an object.

h) Acceleration is the rate at which the velocity changes.

h. TRUE - This is the very definition of acceleration. Know this one - its the beginning point of all our thoughts about acceleration.

h. The value of g on Earth is approximately 9.8 m/s2.

h. TRUE - Yes! Know this one!

i) An object that is accelerating is moving fast.

i. FALSE - Accelerating objects are not necessarily moving fast; they are merely changing how fast they are moving (or the direction they are moving).

i) An object which is slowing down is represented by a line on a velocity-time graph which is moving in the downward direction.

i. FALSE - An object which is slowing down has a velocity which is approaching 0 m/s. And as such, on a v-t graph, the line must be approaching the v=0 m/s axis.

An object has a kinetic energy of 40 J. If its speed were twice as much, then its kinetic energy would be 80 J.

i. FALSE - Kinetic energy is directly related to the square of the speed of an object. So a doubling of the speed would result in a quadrupling of the kinetic energy - the new KE would be 160 J.

The symbol g stands for the force of gravity.

i. FALSE - Nope. A careful physics teacher will never call g the force of gravity. g is known as the acceleration of gravity. It might be best to call it the acceleration caused by gravity. When it comes to the force of gravity, we have yet another symbol for that - Fgrav. But that's a topic to be discussed in a later unit.

j) An object that is accelerating will eventually (if given enough time) be moving fast.

j. FALSE - If the accelerating object is slowing down, then it will eventually stop and not reach a fast speed. And if that doesn't convince you, then consider an object that is accelerating by moving in a circle at constant speed forever; it will accelerate the entire time but never being going any faster than at the beginning.

Object A has a mass of 1 kg and a speed of 2 m/s. Object B has a mass of 2 kg and a speed of 1 m/s. Objects A and B have the same kinetic energy.

j. FALSE - When it comes to kinetic energy, speed is doubly important (recall v2). So in this case, object A would have more kinetic energy. Doing the calculation yields 2 J for object A and 1 J for object B.

k) An object that is moving rightward has a rightward acceleration.

k. FALSE - If an object is moving rightward and slowing down, then it would have a leftward acceleration.

An object can never have a negative kinetic energy.

k. TRUE - Kinetic energy is determined by the equation 0.5•m•v2. the quantity m is always positive. And even if v is negative, v2 will always be positive. Therefore, kinetic energy can never be a negative value.

A falling object always gains kinetic energy as it falls.

l. FALSE - If an object is falling at a constant velocity (i.e., the air resistance force equals the downward force of gravity), then there is not an increase in kinetic energy. It is true however that free-falling objects always increase their kinetic energy as they fall.

An object that is moving rightward and speeding up has a rightward acceleration.

l. TRUE - If an object is speeding up, then the direction of the acceleration vector is in the direction which the object is moving.

m) An object that is moving upwards and slowing down has an upwards acceleration.

m. FALSE - If an object is slowing down, then the acceleration vector is directed opposite the direction of the motion; in this case the acceleration is directed downwards.

A 1-kg object is accelerated from rest to a speed of 2.0 m/s. This object gains 4.0 Joules of kinetic energy.

m. FALSE - The kinetic energy increases from 0 J to 2 J (0.5•1•22); that's an increase by 2 J.

If work is done on an object by a non-conservative force, then the object will either gain or lose kinetic energy.

n. FALSE - Such an object will definitely gain or lose mechanical energy but not necessarily kinetic energy.