Deck 7: Energy and Its Conservation

Question 1

Which of these quantities, if any, is measured by the same unit as work?

A)force
B)power
C)energy
D)none

Answer 1

energy

Question 2

Two skiers starting from the same point go downhill without breaking. They choose two different paths to the bottom of the slope: One goes straight down the steepest slope, the other chooses the longer less steep path. Which skier will have the higher speed at the bottom?

A)The one going down the steeper path.
B)The one going down the longer path.
C)Both will have the same speed at the bottom.
D)We cannot answer this without knowing their masses.

Answer 2

Both will have the same speed at the bottom.

Question 3

You are walking a dog on a leash, pulling it with a force of 40 N. The leash is continuously acting on the dog's body at an angle of 115° with respect to the direction of motion. Which of the following is the value for the work of the tension in the leash after 100 m?

A)+1690 J
B)-1690 J
C)+3625 J
D)-3625 J

Answer 3

-1690 J

Question 4

When a bowling ball rolls along the track, which of the following quantities most influences the amount of kinetic energy of the ball?

A)size
B)speed
C)weight
D)mass

Question 5

You want to climb to the top of a wall that is 2.0 m high, using a ramp. You have a choice of several arrangements for the ramp regarding the length and the angle. Which of the following arrangements would you choose to climb to the top so that you do the least work?

A)2.8 m at 45°
B)4.0 m at 30°
C)11.5 m at 10°
D)The length makes no difference.

Question 6

Figure 7.1
Which sketch of the system and its interaction with the environment represents a closed system?

A)sketch a
B)sketch b
C)sketch c
D)sketch d

Question 7

Figure 7.1
Which expression describes mechanical equilibrium?

A) F = 0
B) F = const
C)ma = const
D)a = const

Question 8

Figure 7.1
Which of the following expresses the unit of work called the Joule?

A)N m
B)kg m/s²
C)kg m²/s
D)kg m/s

Question 9

When an object does work on the environment, which of these statements is correct about speed and kinetic energy?

A)The object becomes slower and kinetic energy decreases.
B)The object becomes faster and kinetic energy decreases.
C)The object becomes slower and kinetic energy increases.
D)The object becomes faster and kinetic energy increases.

Question 10

During your workout, you are lowering a dumbbell with a constant velocity to a vertical distance h. How much work do you do?

A)+mgh
B)-mgh
C)+2mgh
D)zero

Question 11

A child and a parent are running. The ratio of their masses is 1:2, and the ratio of their speeds is 3:1. What is the ratio of their kinetic energies?

A)2:9
B)3:2
C)9:2
D)9:4

Question 12

You throw a 0.50 kg ball vertically up with initial speed 15.0 m/s and catch it with your hand at the same height. What is the kinetic energy of the ball when it hits your hand at the end?

A)0 J
B)3.8 J
C)4.9 J
D)56 J

Question 13

An apple hits you and bounces back with half its original speed. What is the percentage of kinetic energy that the apple lost in the collision?

A)20%
B)25%
C)50%
D)75%

Question 14

A ballet dancer lifts his partner up to the height h and lowers her again to the same height he started from. Which of these statements describes the total work of gravity during this move?

A)It depends on the value of the height h.
B)It is zero.
C)It depends on the path chosen for the motion.
D)It depends on the mass of his partner.

Question 15

When positive work is done on a system, which of these statements best describes the way energy changes?

A)Kinetic and potential energy both increase.
B)Kinetic and potential energy both decrease.
C)Potential energy decreases.
D)Kinetic energy increases.

Question 16

You want to climb to the top of a 2.0 m high wall using a ramp. You have a choice of several arrangements for the ramp regarding the length and the angle. Which one of these arrangements would you choose in order to climb to the top with the least "energy consumption in time" (power)?

A)2.8 m at 45°
B)4.0 m at 30°
C)11.5 m at 10°
D)The length makes no difference.

Question 17

You throw a ball vertically up in the air. Which type of energy increases and which decreases?

A)Kinetic increases and gravitational potential decreases.
B)Kinetic decreases and gravitational potential decreases.
C)Kinetic increases and gravitational potential increases.
D)Kinetic decreases and gravitational potential increases.

Question 18

A body with a speed v has energy E. A second identical body with speed v is moving in the opposite direction. What is the kinetic energy of the second body?

A) E
B)- E
C)1/9 E
D)-1/9 E

Question 19

You lift a 10 kg bag of flour vertically upward at a constant speed of 0.1 m/s. You lift the bag a distance of 0.5 m. How much work have you done?

A)4.9 J
B)9.4 J
C)49 J
D)94 J

Question 20

A body goes to the left, but one of the forces pushes it to the right, opposing its motion. Which of the following is correct?

A)Positive work is done on an object.
B)Positive work is done by the object.
C)Negative work is done on an object.
D)Negative work is done by the object.

Question 21

A standard man with a mass of 70 kg runs on a treadmill at a constant velocity of 5 m/s, exerting a constant force F_man = 600 N up the 25° slope of a treadmill, for 6 minutes. The coefficient of kinetic friction of a treadmill is ì = 0.40. What is the work done by the standard man?

Question 22

Energy enables a system to do work.

Question 23

You are holding in your hands a heavy picture frame that weighs 150 N, and getting tired of holding it. How much work did you do on the frame while holding it up?

Question 24

In elastic collisions, both momentum and kinetic energy are conserved; but in inelastic collisions, only momentum is conserved.

Question 25

A closed system can exchange only matter and not energy with the environment.

Question 26

Figure 7.2
Work depends on displacement vector and force vector, but it is a scalar. Explain.

Question 27

An object of mass 5 kg is moving at a speed of 6 m/s and hits a stationary ball with a mass of 10 kg. What percentage of its kinetic energy does the object with a mass of 5 kg transfer to the 10 kg ball?

A)11%
B)50%
C)89%
D)100%

Question 28

Give an example of negative work.

Question 29

During a round trip, the work of a conservative force is zero.

Question 30

The work is negative when work is done on the system.

Question 31

A meteorite falling toward Earth is losing potential energy and gaining kinetic energy.

Question 32

Figure 7.2
Describe characteristics of each system in Fig. 7.2. Which one of the systems is impossible?

Question 33

When an object does work on the environment, it gains kinetic energy and accelerates.

Question 34

Two particles collide elastically. The particles have different masses but equal momenta. In a collision, the system lost 50% of its kinetic energy.

Question 35

If we filmed an elastic collision and showed it backward, we would not be able to tell the difference.

Question 36

You throw a 0.50 kg ball vertically up with initial speed of 15.0 m/s. What is the kinetic energy of the ball at the highest point?

A)0 J
B)3.8 J
C)4.9 J
D)56 J

Question 37

Work is energy transferred in or out of a system, and is equal to the amount of energy change.

Question 38

Work depends on a displacement vector and a force vector; therefore, it depends on a direction.

Question 39

Which of these statements correctly describes an elastic collision between two objects?

A)Neither momentum nor kinetic energy is conserved.
B)Both momentum and kinetic energy are conserved.
C)Momentum is conserved, but kinetic energy is not.
D)Kinetic energy is conserved, but not momentum.

Question 40

It is not possible to have achieve displacement but have work of a force zero.

Question 41

Figure 7.4 The figure shows an object attached to a string, making a pendulum.
A ball of mass 5 kg is attached to a string of length 2 m, as shown in Fig. 7.4. If we release the ball at an angle of 85°, and let it hit a stationary ball of mass 10 kg at the bottom of its trajectory, what is the speed of each ball after the collision? Assume the collision is elastic.

Question 42

Figure 7.3
You are exercising with a 5 kg dumbbell, lifting it vertically up 40 cm and bringing it back to the same position (Fig. 3). How much work is done on the dumbbell by you, and how much by gravitational force?

Question 43

Canadian golf champion Jason Zuback achieved record speed of the ball, hitting it to 328 km/h. If he hit a 46 g ball at an angle of 45° for maximal range, what is the kinetic energy at the top of the trajectory? How does that compare to the kinetic energy when the ball was hit?

Question 44

The driver reduced the speed of his car to half when he saw the sign for school area. To what percentage of the initial, was kinetic energy reduced?

Question 45

A 10 kg steel ball with an initial speed of 12 m/s hits a stationary 20 kg ball. As the result of an elastic collision, the lighter ball moves backward. What are the velocities of the two balls after the collision?

Question 46

Figure 7.3
An acrobatic skier pushes himself down a semi-circular ramp and jumps directly upward from the edge on the opposite side. If he flies upward to a height of 1.4 m above the ramp's end, what was his initial velocity?

Question 47

You throw a 0.5 kg ball vertically upward with initial speed of 10 m/s. What are the kinetic, potential, and total energies at the top of the trajectory (highest point)? How high is that point from your hand?

Question 48

Figure 7.4 The figure shows an object attached to a string, making a pendulum.
A ball of mass 0.5 kg is attached to a string of length L = 1.5 m, as shown in Fig. 7.4. If we release the ball at an angle of 45°, what is its speed at the bottom of its trajectory?