Deck 7: Work and Kinetic Energy

Question 1

Power is a vector quantity.

Answer 1

False

Question 2

The kinetic energy of an object increases linearly as the velocity of the object increases.

Answer 2

False

Question 3

A person applies a constant force of 20 N to a rock of mass 1000 kg, for a total of 20 seconds. What is the work done by this person if the rock does not move at all by this applied force?

A) 1000 J
B) 2000 J
C) 20,000 J
D) 0 J
E) 400 J

Answer 3

0 J

Question 4

If you walk 5.0 m horizontally forward at a constant velocity carrying a 10 N object, the amount of work you do is

A) more than 50 J.
B) equal to 50 J.
C) less than 50 J, but more than 0 J.
D) zero.

Question 5

If you push twice as hard against a stationary brick wall, the amount of work you do

A) quadruples.
B) doubles.
C) is cut in half.
D) remains constant but non-zero.
E) remains constant at zero.

Question 6

Kinetic energy is a scalar quantity.

Question 7

Can work be done on a system if there is no motion?

A) Yes, since motion is only relative.
B) Yes, if the sum of the external forces is zero.
C) Yes, if an external force is acting on the system.
D) No, since a system which is not moving has no energy.
E) No, since work involves a non-zero displacement.

Question 8

Two men, Joel and Jerry, push against a wall. Jerry stops after 10 min, while Joel is able to push for 5.0 min longer. Compare the work they do.

A) Joel does 75% more work than Jerry.
B) Joel does 50% more work than Jerry.
C) Jerry does 50% more work than Joel.
D) Joel does 25% more work than Jerry.
E) Neither of them do any work.

Question 9

The work required to stretch a spring changes linearly with the amount of stretch.

Question 10

Work done by a force on an object can be a positive or a negative quantity.

Question 11

State the work-energy theorem.

Question 12

Work done by STATIC FRICTION is always

A) parallel to the surface.
B) perpendicular to the surface.
C) positive.
D) negative.
E) zero.

Question 13

Work is a vector quantity.

Question 14

A simple pendulum, consisting of a mass m and a string of length L, swings upward, making an angle θ with the vertical. The work done by the tension force is

A) zero.
B) mgL.
C) mgL cos θ.
D) mgL sin θ.
E) mgL tan θ.

Question 15

A person carries a mass of 10 kg and walks along the +x-axis for a distance of 100 m with a constant velocity of 2 m/s. What is the work done by this person?

A) 0 J
B) 20 J
C) 200 J
D) 1000 J
E) None of the other choices is correct.

Question 16

Kinetic energy is proportional to mass.

Question 17

Kinetic energy is proportional to speed.

Question 18

If the net work done on an object is positive, then the object's kinetic energy

A) decreases.
B) remains the same.
C) increases.
D) is zero.
E) cannot be determined without knowing the object mass.

Question 19

A person applies a constant force on an object of mass 20 kg that causes the object to move horizontally at a constant speed of 0.20 m/s through a distance of 0.80 m. What is the work done on the object?

A) 160 J
B) 10 J
C) 16 J
D) 0 J
E) Cannot be determined without knowing the magnitude of the applied force.

Question 20

A constant force is applied to an object that causes a certain displacement. If the angle between the force and the displacement is 135°, the work done by this force is

A) positive.
B) negative.
C) 0 J.
D) Cannot be determined without knowing the magnitude of the displacement.
E) Cannot be determined without knowing the magnitude of the applied force.

Question 21

Compared to yesterday, you did 3 times the work in one-third the time. To do so, your power output must have been

A) the same as yesterday's power output.
B) one-third of yesterday's power output.
C) 3 times yesterday's power output.
D) 9 times yesterday's power output.
E) 3⁴ times yesterday's power output.

Question 22

FIGURE 7-1
Which of the graphs in Figure 7-1 represents a spring which gets less stiff the more it is stretched?

A) graph a
B) graph b
C) graph c
D) graph d
E) none of these

Question 23

A truck has four times the mass of a car and is moving with twice the speed of the car. If K_t and K_c refer to the kinetic energies of truck and car respectively, it is correct to say that

A) K_t = 16K_c.
B) K_t = 4K_c.
C) K_t = 2K_c.
D) K_t = K_c.
E) K_t = K_c.

Question 24

An object hits a wall and bounces back with half of its original speed. What is the ratio of the final kinetic energy to the initial kinetic energy?

A) 1/2
B) 1/4
C) 2
D) 4
E) 8

Question 25

If the net work done on an object is negative, then the object's kinetic energy

A) decreases.
B) remains the same.
C) increases.
D) is zero.
E) cannot be determined without knowing the object mass.

Question 26

FIGURE 7-1
Which of the graphs in Figure 7-1 illustrates Hooke's Law?

A) graph a
B) graph b
C) graph c
D) graph d
E) none of these

Question 27

In the SI system of units, power has the same units as

A) Js/m.
B) Jm/s.
C) Nm/s.
D) W/m.
E) W/s.

Question 28

As compared to Jack, Jill does twice the work in half the time. Jill's power output is

A) the same as Jack's power output.
B) one-fourth as much as Jack's power output.
C) one-half as much as Jack's power output.
D) twice Jack's power output.
E) four times Jack's power output.

Question 29

If the net work done on an object is zero, then the object's kinetic energy

A) decreases.
B) remains the same.
C) increases.
D) is zero.
E) cannot be determined without knowing the object mass.

Question 30

On a force vs. position graph, the area under curve is a representation of

A) force.
B) position.
C) kinetic energy.
D) potential energy.
E) work.

Question 31

The ratio of the final kinetic energy to the initial kinetic energy of an object is one half. If the initial velocity of the object is 10 m/s, what is the final velocity?

A) 20 m/s
B) 10 m/s
C) 7.1 m/s
D) 2.7 m/s
E) 1.5 m/s

Question 32

John's mass is half the mass of Jill. They both start walking and John moves twice as fast as Jill. What is the ratio of the kinetic energy of Jill to the kinetic energy of John?

A) 4
B) 2
C) 1
D) 1/2
E) 1/8

Question 33

A 4.0 kg mass is moving with speed 2.0 m/s. A 1.0 kg mass is moving with speed 4.0 m/s. Both objects encounter the same constant braking force, and are brought to rest. Which object travels the greater distance before stopping?

A) the 4.0 kg mass
B) the 1.0 kg mass
C) Both travel the same distance.
D) Cannot be determined from the information given.

Question 34

You slam on the brakes of your car in a panic, and skid a certain distance on a straight, level road. If you had been traveling twice as fast, what distance would the car have skidded, under the same conditions?

A) It would have skidded 4 times farther.
B) It would have skidded twice as far.
C) It would have skidded 1.4 times farther.
D) It would have skidded one half as far.
E) It is impossible to tell from the information given.

Question 35

A block of mass m is pushed against a spring of spring constant k. The spring is compressed by a distance d, the block is then released. It is launched by the spring along a horizontal frictionless surface with a final speed v. A second block, this one having mass 4m is pushed against the same spring by distance 6d and released. What is the final speed of the block in this case?

A) v
B) 2v
C) 3v
D) 4v
E) 5v

Question 36

A car originally at rest has its speed increased to a value v in a period of 5 seconds. The work performed in this part of the motion is W₁. Over the next 5 seconds the speed of the car is increased to 2v. In the second part of the motion the work performed is W₂. It is correct to say that:

A) W₁ = W₂.
B) W₁ = W₂.
C) W₁ = 2W₂.
D) W₁ = 3W₂.
E) W₁ = 4W₂.

Question 37

Consider a plot of the displacement (x) vs. applied force (F) for an ideal elastic spring. The slope of the curve would be

A) the spring constant.
B) the reciprocal of the spring constant.
C) the acceleration of gravity.
D) the reciprocal of the acceleration of gravity.
E) the reciprocal of the displacement.

Question 38

A force produces power P by doing work W in a time T. What power will be produced by a force that does six times as much work in half as much time?

A) 12P
B) 6P
C) P
D) P
E) P

Question 39

FIGURE 7-1
Describe the type of spring "constant" needed to produce a constant restoring force like curve (a) in Figure 7-1.

A) k must vary as the stretch squared.
B) k must be a real constant.
C) k must vary inversely with stretch.
D) k must vary proportional to stretch.
E) none of these

Question 40

A block of mass m is pushed against a spring of spring constant k. The spring is compressed by a distance d, the block is then released. It is launched by the spring along a horizontal frictionless surface with a final speed v. A second block, this one having mass 4m is pushed against the same spring and released, gaining a final speed 2v. By what distance was the spring compressed in the second case?

A) d
B) 2d
C) 4d
D) 16d
E) 25d

Question 41

Daisy raises 10.Kg to a height of 2.5 meters in 2.0 seconds.
(a) How much work did she do?
(b) How much power was expended?
(c) If she raises it in 1.0 s rather than 2.0 s, how do the work and power change?

Question 42

An object of mass 10.0 kg is released from the top of an inclined plane which makes an angle of inclination of 30.0° with the horizontal. The object slides along the inclined plane. The questions refer to the instant when the object has traveled through a distance of 2.00 m measured along the slope. The coefficient of kinetic friction between the mass and the surface is 0.200. Use g = 10.0 m/s².
(a) How much work is done by the net force on the mass?
(b) How much work is done by the force of friction?

Question 43

As plant engineer, Donna purchases an electrical motor which has an output of 1500. hp. If it requires 1.20 Megawatts of electricity, what is the efficiency of the motor?

Question 44

FIGURE 7-2 Three applied forces, F₁ = 20.0 N, F₂ = 40.0 N, and F₃ = 10.0 N act on an object with a mass of 2.00 kg which can move along an inclined plane as shown in the figure. The questions refer to the instant when the object has moved 0.600 m along the surface of the inclined plane in the upward direction. Neglect friction and use g = 10.0 m/s².
Refer to Figure 7-2. What is the amount of work done by force F₁ as the object moves up the inclined plane?

A) 10.0 J
B) 11.0 J
C) 12.0 J
D) 16.0 J
E) 0 J

Question 45

An airplane flies 160 km at a constant altitude in a direction 30.0° north of east. A wind is blowing that results in a net horizontal force on the plane due to the air of 2.40 kN in a direction 10.0° south of west. How much work is done by the air on the plane?

A) -3.61 × 10⁸ J
B) -1.31 × 10⁸ J
C) -2.94 × 10⁸ J
D) 2.94 × 10⁸ J
E) 1.15 × 10⁸ J

Question 46

FIGURE 7-2 Three applied forces, F₁ = 20.0 N, F₂ = 40.0 N, and F₃ = 10.0 N act on an object with a mass of 2.00 kg which can move along an inclined plane as shown in the figure. The questions refer to the instant when the object has moved 0.600 m along the surface of the inclined plane in the upward direction. Neglect friction and use g = 10.0 m/s².
Refer to Figure 7-2. What is the amount of work done by force F₂ as the object moves up the inclined plane?

A) 0 J
B) 12.0 J
C) 16.0 J
D) 24.0 J
E) 20.8 J

Question 47

How many joules of energy are used by a 2.0 hp motor that runs for 1.0 hr?

Question 48

A 5.00-kg box slides 4.00 m across the floor before coming to rest. What is the coefficient of kinetic friction between the floor and the box if the box had an initial speed of 3.00 m/s?

A) 1.13
B) 0.587
C) 0.115
D) 0.229
E) 0.267

Question 49

FIGURE 7-3
An object of mass = 2 kg is pulled by a constant force F = 4 N for a horizontal distance of 2 m. (Refer to Figure 7-3.) What is the work done along the +x-axis? Neglect friction.

A) 9 J
B) 8 J
C) 5 J
D) 7 J
E) 2 J

Question 50

FIGURE 7-4 A constant force of 100 N is applied to a mass of 40.0 kg as shown in Figure 7-4. The force causes a displacement of 20.0 m to this mass along the + x-axis. Neglect friction.
What is the work done by the component of the force in Figure 7-4 along the + y-axis?

A) 1680 J
B) 1530 J
C) 1520 J
D) 0 J
E) 2000 J

Question 51

A horizontal force of 40 N pushes an object of mass 5.0 kg up an inclined plane through a distance of 1.6 m measured along the plane. The plane is inclined at an angle of 30° to the horizontal. Neglect friction and use g = 10 m/s².
(a) What is the work done by the horizontal force on the object?
(b) What is the work done by the net force on the object?
(c) What is the work done by the normal force on the object?

Question 52

An object of mass 10.0 kg is released from the top of an inclined plane which makes an angle of inclination of 30.0° with the horizontal. The object slides along the inclined plane. The questions refer to the instant when the object has traveled through a distance of 2.00 m measured along the slope. The coefficient of kinetic friction between the mass and the surface is 0.200. Use g = 10 m/s². How much work is done by gravity?

A) 100 J
B) 20.0 J
C) 0 J
D) 10.0 J
E) 131 J

Question 53

FIGURE 7-4 A constant force of 100 N is applied to a mass of 40.0 kg as shown in Figure 7-4. The force causes a displacement of 20.0 m to this mass along the + x-axis. Neglect friction.
What is the work done by the component of the force in Figure 7-4 along the +x-axis?

A) 1680 J
B) 1530 J
C) 1520 J
D) 2000 J
E) 2000 J.

Question 54

An object of 1.0 kg mass is pulled up an inclined plane by a constant force of 10 N that causes a displacement of 0.50 m. The angle of inclination with the horizontal is 30°. Neglect friction and use g = 10 m/s². What is the work done by the net force on the object along the inclined plane?

A) 4.3 J
B) 4.3 kg m/s²
C) 0 J
D) 2.5 J

Question 55

I left five 100. watt bulbs burning in my basement for two weeks. If electricity costs me 10.¢/kw-h, how much did the electricity cost (to the nearest dollar) to leave these bulbs on and how many Joules of electrical energy did they consume?

Question 56

Lisa runs up 4 flights of stairs in 22. seconds. She weighs 510. Newtons. If each flight rises 310. cm:
(a) What is her change in potential?
(b) What average power (watts) was required during the 22. s?
(c) What minimum horsepower motor would be required to do this?

Question 57

An object of 1.0 kg mass is pulled up an inclined plane by a constant force of 10 N that causes a displacement of 0.50 m. The angle of inclination with the horizontal is 30°. Neglect friction and use g = 10 m/s². What is the work done by the 10 N force on the object along the inclined plane?

A) 4.3 J
B) 4.3 kg m/s²
C) 5.0 J
D) 2.5 J

Question 58

A constant force of 20 N is applied to an object of mass 8.0 kg at an angle of 25° with the horizontal. What is the work done by this force on the object if it causes a displacement of 2.0 m along the horizontal direction?

A) 40 J
B) 0 J
C) 36 J
D) 17 J
E) 19 J

Question 59

A student slides her 80.0-kg desk across the level floor of her dormitory room a distance 4.00 m at constant speed. If the coefficient of kinetic friction between the desk and the floor is 0.400, how much work did she do?

A) 128 J
B) 3.14 kJ
C) 26.7 J
D) 1.26 kJ
E) 24.0 J

Question 60

FIGURE 7-2 Three applied forces, F₁ = 20.0 N, F₂ = 40.0 N, and F₃ = 10.0 N act on an object with a mass of 2.00 kg which can move along an inclined plane as shown in the figure. The questions refer to the instant when the object has moved 0.600 m along the surface of the inclined plane in the upward direction. Neglect friction and use g = 10.0 m/s².
Refer to Figure 7-2. What is the amount of work done by the force F₃ as the object moves up the inclined plane?

A) 12.0 J
B) 16.0 J
C) 20.8 J
D) 0 J
E) 24.0 J

Question 61

FIGURE 7-5
An object is under the influence of a force as represented by the force vs. position graph in Figure 7-5. What is the work done as the object moves from 0 m to 4 m?

A) 20 J
B) 30 J
C) 0 J
D) 40 J
E) 70 J

Question 62

How much energy is needed to change the speed of a 1600 kg sport utility vehicle from 15.0 m/s to 40.0 m/s?

A) 1.10 MJ
B) 10.0 kJ
C) 20.0 kJ
D) 40.0 kJ
E) 0.960 MJ

Question 63

4.0 J of work are performed in stretching a spring with a spring constant of 2500 N/m. How much is the spring stretched?

A) 3.2 cm
B) 3.2 m
C) 0.3 cm
D) 5.7 m
E) 5.7 cm

Question 64

A weight of 200 N is hung from a spring with a spring constant of 2500 N/m and lowered slowly. How much will the spring stretch?

A) 4.00 cm
B) 6.00 cm
C) 8.00 cm
D) 10.0 cm
E) 12.0 cm

Question 65

A 1.0-kg object moving in a certain direction has a kinetic energy of 2.0 J. It hits a wall and comes back with half its original speed. What is the kinetic energy of this object at this point?

A) 2.0 J
B) 1.0 J
C) 0.50 J
D) 0.25 J
E) 4.0 J

Question 66

A spring with a spring constant of 2500 N/m. is stretched 4.00 cm. What is the work required to stretch the spring?

A) 4.00 J
B) 0 J
C) 1.00 J
D) 3.00 J
E) 2.00 J

Question 67

Which object has larger kinetic energy: a 500-kg object moving with a speed of 40 m/s or 1000-kg object moving at 20 m/s?

A) the 500-kg object moving at 40 m/s
B) Both have same kinetic energy.
C) the 1000-kg object moving at 20 m/s
D) Kinetic energy is independent of mass.
E) Kinetic energy is independent of speed.

Question 68

A boy throws a ball to another boy who throws it back with half the original speed. What is the ratio of the final kinetic energy to the initial kinetic energy of the ball?

A) 0.25
B) 0.50
C) 2.00
D) 0.75
E) 1.0

Question 69

FIGURE 7-6
An object is under the influence of a force as represented by the force vs. position graph as shown in Figure 7-6. What is the work done as it moves from 0 to 10 m?

A) 25 J
B) 50 J
C) 100 J
D) 80 J
E) 125 J

Question 70

How large a force is required to accelerate a 1600 kg car from rest to a speed of 25 m/s in a distance of 200 m?

A) 1600 N
B) 0 N
C) 200 N
D) 400 N
E) 2500 N

Question 71

FIGURE 7-5
An object is under the influence of a force as represented by the force vs. position graph in Figure 7-5. What is the work done as the object moves from 6 m to 12 m?

A) 20 J
B) 30 J
C) 0 J
D) 40 J
E) 70 J

Question 72

FIGURE 7-5
An object is under the influence of a force as represented by the force vs. position graph in Figure 7-5. What is the work done as the object moves from 4 m to 6 m?

A) 20 J
B) 30 J
C) 0 J
D) 40 J
E) 70 J

Question 73

If the work done to stretch a spring by 4.0 cm is 6.0 J, what is the spring constant?

A) 300 N/m
B) 3000 N/m
C) 3500 N/m
D) 7500 N/m
E) 6000 N/m

Question 74

FIGURE 7-6
An object is under the influence of a force as represented by the force vs. position graph as shown in Figure 7-6. What is the work done as it moves from 10 to 15 m?

A) 25 J
B) 50 J
C) 100 J
D) 80 J
E) 125 J

Question 75

The ratio of the kinetic energy of object A to the kinetic energy of the object B is 2: 1. The mass of object A is 2.0 kg and the mass of object B is 4.0 kg. If object A has a speed of 4.0 m/s, what is the speed of object B?

A) 8.0 m/s
B) 6.0 m/s
C) 4.0 m/s
D) 1.4 m/s
E) 2.0 m/s

Question 76

An object of mass 10.0 kg is initially at rest. A 100 N force causes it to move horizontally through a distance of 6.00 m. What is the change in the kinetic energy of this object?

A) 0 J
B) 200 J
C) 60.0 J
D) 600 J
E) 1000 J

Question 77

The ratio of the mass of object A to the mass of object B is 2: 1 and the ratio of their speeds is 1:
2 in the same order. What is the ratio of their kinetic energies?

A) 1:2
B) 2:1
C) 3:1
D) 1:3
E) 1:4

Question 78

A 20.0-N weight slides down a rough inclined plane which makes an angle of 30.0° with the horizontal. The weight starts from rest and gains a speed of 15.0 m/s after sliding 150 m. How much work is done against friction? Use g = 10 m/s².

A) 1270 J
B) 1500 J
C) -1500 J
D) 229 J
E) -229 J

Question 79

The ratio of kinetic energy of object A to the kinetic energy of object B is 2: 1. Object A is moving with a speed of 6.0 m/s and object B is moving with a speed of 2.0 m/s. If the mass of object A is 4.0 kg then what is the mass of object B?

A) 2.0 kg
B) 4.0 kg
C) 8.0 kg
D) 18 kg
E) 16 kg

Question 80

An object of mass m moving in a certain direction has a kinetic energy of 4.0 J. It hits a wall and comes back with half its original kinetic energy. If the speed of the object on return is 2.0 m/s, what is the mass of the object?

A) 2.0 kg
B) 2.8 kg
C) 3.8 kg
D) 1.0 kg
E) 4.0 kg