Topic 2.3: Work, Energy, Power Flashcards Preview

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Flashcards in Topic 2.3: Work, Energy, Power Deck (57):
1

When a car is moving at a speed v in a straight line the total force opposing its motion is F. The product Fv is a measure of the A. power developed by the car. B. rate of change of momentum of the car. C. work done by the car against the frictional forces. D. rate of change of kinetic energy of the car.

A. power developed by the car.

2

An object of mass m1 has a kinetic energy K1. Another object of mass mhas a kinetic energy K2 . If the momentum of both objects is the same, the ratio K1/K2 is equal to
 

A. m2/m1

B. m1/m2

C. (m2/m1)1/2

D. (m1/m2)1/2

A. m2/m1

3

Which one of the following units is a unit of energy?
A. eV
B. W s−1
C. W m−1
D. Nms-1

A. eV

4

A rocket is fired vertically. At its highest point, it explodes. Which one of the following describes what
happens to its total momentum and total kinetic energy as a result of the explosion?

                          Total Momentum               Total Kinetic Energy

A.                       unchanged                         increased

B.                       unchanged                         unchanged

C.                       increased                            increased

D.                       increased                            unchanged

A. total momentum is unchanged and total kinetic energy increases

5

The graph below shows the variation with displacement d of the force F applied by a spring on a cart.

The work done by the force in moving the cart through a distance of 2 cm is
A. 10 x 10-2 J
B. 7 x 10-2J
C. 5 x 10-2J
D. 2.5 x 10-2 J

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C. 5 x 10-2J

6

Which of the following quantities are conserved in an inelastic collision between two bodies?

                       Total linear momentum                  Total kinetic energy                             on the bodies                                 on the bodies

A.                    yes                                                    yes

B.                    yes                                                    no

C.                    no                                                     yes

D.                    no                                                     no

B. total linear mometum is the only one conserved in an inelastic collision between two bodies

7

The diagram below shows the variation with displacement x of the force F acting on an object in the direction of the displacement.

Which area represents the work done by the force when the displacement changes from to x1 to x2?
A. QRS
B. WPRT
C. WPQV
D. VQRT

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D. VQRT

8

An engine takes in an amount E of thermal energy and, as a result, does an amount W of useful work. An amount H of thermal energy is ejected. The law of conservation of energy and the efficiency of the engine are given by which of the following?

                          Law of conservation of energy                Effieciency

A.                       E= W + H                                                    W

B.                       E= W + H                                                     W/E

C.                       E + H = W                                                    W/H

D.                       E + H + W                                                    W/(E - H)

B. law of conservation of energy is represented by E = W + H, and effieciency is represented by W/E

9

A machine lifts an object of weight 1.5 x 103 N to a height of 10 m. The machine has an overall efficiency of 20 %. The work done by the machine in raising the object is

A. 3.0 x 103J

B. 1.2 x 104J

C. 1.8 x 104J

D. 7.5 x 104J

D. 7.5 x 104J

10

Joe is standing on the surface of a frozen pond and he throws a ball horizontally. Considering Joe and the ball together, which one of the following correctly describes the change in the magnitude of the
momentum and the change in the kinetic energy of Joe and the ball immediately after the ball is thrown?

                          Magnitude of momentum          Kinetic energy of                             of Joe and ball                                Joe and ball

A.                       no change                                  increases

B.                       increases                                    increases

C.                       no change                                  no change

D.                      increases                                     no change

A. The is no change in magnitude of momentum of Joe and the ball, but the kinetic energy of Joe and the ball increases

11

The variation with time of the vertical speed of a ball falling in air is shown below.

During the time from 0 to T, the ball gains kinetic energy and loses ∆Ep gravitational potential energy. Which of the following statements is true?
 A. ∆Ep is equal to the gain in kinetic energy.
 B. ∆Ep is greater than the gain in kinetic energy.
 C. ∆Ep is equal to the work done against air resistance.
 D. ∆Ep is less than the work done against air resistance. 

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B.The change in gravitational potental energy is greater than the gain in kinetic energy.

12

The diagram below represents energy transfers in an engine.

The efficiency of the engine is given by the expression

A. Ew/Ein

B. Ew/Eout

C. Eout/Ein

D. Eout/Ew

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C. efficiency is the energy put out over the energy in

13

An object of mass m is initially at rest. An impulse I acts on the object. The change in kinetic energy of the object is

A. I2/2m

B. I2/m

C. I2m

D. 2I2m

A. The change in kinetic energy is equal tothe impulse squared divided by 2 time the mass

14

The graph below shows the variation with displacement d of the force F acting on a particle

The area that represents the work done by the force between d = 0 and d = dmax is

A. s − r.
B. r.

C. s.
D. s + r.

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D. s + r

15

A spring is compressed by a force F.

For a compression e, the force F is given by F = ke. When the compression force is removed, the spring returns to its original length in time t. The best estimate for the power developed by the spring 
during its expansion is

A. ke/2t

B. ke/t

C. ke2/2t

D. ke2/t

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C. ke2/2t

16

A     force    stretches    a    wire    that    is    fixed    at    one    end.   The    value    of    this     force    increases     from    zero    to    a    
maximum value and then returns to zero. The graph below shows the variation with force F of the extension x of the wire.

Which area, or areas, represents the net work done on the wire by the force?
 A. Area P
 B. Area Q
 C. Area R
 D. Area Q and area R

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B. Area Q

17

A body moving along a straight-line has mass 3.0 kg and kinetic energy 24J. The motion is then opposed by a net force of 4.0 N. The body will come to rest after travelling a distance of
 A. 2.0 m.
 B. 6.0 m.
 C. 8.0 m.
 D. 12 m.

B. 6.0m

18

Two objects collide inelastically. For this system of two objects
 A. only momentum is conserved.
 B. only kinetic energy is conserved.
 C. both momentum and kinetic energy are conserved.
 D. neither momentum nor kinetic energy are conserved.

A. Only momentum is conserved

19

An object of weight 50 N is dragged up an inclined plane at constant speed, through a vertical height of 12 m. The total work done is 1500 J. The work done against friction is 
 A. 2100 J.
 B. 1500 J.
 C. 900 J.
 D. 50 J.

C. 900 J

20

The graph below shows the variation with load F of the length L of a spring.

Which of the following expressions gives the force per unit extension (the spring constant) of 
the spring?

A. F1/L1

B. F2/L2

C. (F2-F1)/L2

D. (F2-F1)/(L2-L1)

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D. The spring constant is the second force subtracted by the first force divided by the second lenght divided by the first length.

21

The graph below shows the variation with displacement x of the force F acting on an object. The force F always acts in the same direction as the displacement.

At point Q, the displacement is xQ and the force is FQ.
Which of the following gives the work done by the force on the body as the displacement increases from zero to xQ and then returns to zero?

A. Zero
 B. 1/2FQxQ
 C. FQxQ
 D. 2FQxQ

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A. Zero

22

Water flows out from a tank down a pipe, as shown below

The pipe is always full of water. Which of the following gives the change in the kinetic energy and in the gravitational potential energy 
of the water as the water flows down the pipe?

                     kinetic energy                  gravitational potential energy

A.                  constant                           decreases

B.                  constant                           increases

C.                  increases                         decreases

D.                  increases                         increases

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A. the kinetic energy will stay constant while the gravitational potential energy will decrease

23

Engine X is stated to be more powerful than engine Y.
Which of the following is the correct comparison of the engines?
 A. Engine X produces a larger force than engine Y.
 B. Engine X produces more useful energy than engine Y.
 C. Engine X produces more useful energy per unit time than engine Y.
 D. Engine X produces more power for a longer time than engine Y

C. Engine X produces more useful energy per unit time than engine Y.

24

An elevator (lift) of mass m is raised vertically with constant speed v for a time t. The work done on 
the elevator during this time is
 A. mgv.
 B. mgvt.
 C. mgv/t
 D. mgt/v

B. mgvt

25

A force of magnitude F1 accelerates a body of mass m from rest to a speed v. A force of magnitude F2 accelerates a body of mass 2m from rest to a speed 2v. The ratio work done by F2/ work done by F1 is
 A. 2.
 B. 4.
 C. 8.
 D. 16.

C. 8

26

A weight is suspended from a spring. The variation with weight of the length of the spring is shown below.

What is the value of the spring constant (force constant) of the spring?
 A. 0.4 N cm-1
 B. 0.5 N cm-1
 C. 2.0 N cm-1
 D. 2.5 N cm−1

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C. 2.0 N cm-1

27

The point of action of a constant force F is displaced a distance d. The angle between the force and the direction of the displacement is θ , as shown below.

Which one of the following is the correct expression for the work done by the force?
 A. Fd sinθ
 B. Fd sinθ
 C. Fd cosθ
 D. Fd tanθ

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C. Fd cosθ

28

Which one of the following is a true statement about energy?
 A. Energy is destroyed due to frictional forces.
 B. Energy is a measure of the ability to do work.
 C. More energy is available when there is a larger power.
 D. Energy and power both measure the same quantity.

B. Energy is a measure of the ability to do work.

29

A body of mass m and speed v has kinetic energy EK . A second body of mass m/2 moves at speed 2v. The kinetic energy of this second body is
 A. EK/2
 B. EK
C. 2EK 
D. 4EK

C. 2EK

30

A box of mass m is moved horizontally against a constant frictional force f through a distance s at constant speed v. The work done on the box is
A. 0.

B. mgs.
C. 1/2 mv2
D. fs

D. The work done is the force multiplied by the distance.

31

An electric motor, with an input power of 250 W, produces 200 W of mechanical power. The efficiency of the motor is
 A. 20 %.
 B. 25 %.
 C. 55 %.
 D. 80 %.

D. The efficiency is 80%

32

What is work?(2)

work is when a force causes an object to move in the direction of that force; defined as force times change in distance

33

Is work a scalar or vector quantity?

scalar

34

Is energy a scalar or vector quantity?

scalar

35

What are the 3 ways to write the units for work and energy?

J;Nm;Kgm2sec-2

36

What is mechanical energy?(4)

when work is done upon an object, the object gains energy; can be potential or kinetic; sum of kinetic energy plus all other forms of potential energy; ME=PE+KE

37

What is potential energy?(2)

any time the position of an object gives the possibility that it could fall or move it has "potential energy"; has potential to do some work

38

What are the three types of potential energy?

elastic potential energy; gravitational potential energy; chemical energy

39

What are the 2 equations for gravitational potential energy?

GPE=mgh; GPE=Fwh

40

What is work done?

work done is the amount of potential energy gained

41

What is elastic potential energy?(2)

the energy stored in elastic materials as the result of their stretching or compressing; PEspring=(1/2)kx2

42

What are the units for the spring constant?

Nm-1

43

What is kinetic energy?(4)

moving objects have the ability to do work, so they have energy; energy in motion; KE=(1/2)mv2; the work you do on an object is equal to the kinetic energy it gained

44

What is the Law of Conservation of Energy?(5)

total energy of the system is constant; this principle says that in a closed system energy is conserved; MEinitial=MEfinal; energy is only conserved if the collision is perfectly elastic; if collision is inelastic, momentum is conserved, but energy is not

45

What is power?(3)

a measure of how fast work is done; it is the amount of work done divided by the time it takes; P=w/t

46

What are the units for power?(2)

Js-1; W

47

What is a watt?(2)

defined as one joule per second; 1W=1J/1s

48

What is efficiency?(2)

work put out over work put in expressed as a percentage; (Wout/Win)x100

49

A ball falls vertically and bounces off the ground. Immediately before impact with the ground the speed of the ball is u. Immediately after leaving the ground the speed is v. Which of the following expressions is the ratio of kinetic energy lost on collision/kinetic energy immediately before collision ?

A.     v/u
B. 1- (v/u)
C.     (v/u)2

D.     1- (v/u)2

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D

50

A railway engine of mass m moves along a horizontal track with uniform speed v. The total resistive force acting on the engine is F.

Which of the following is the power of the engine?

A.    F/mv
B.     Fv
C.  mv/F
D.  v/F

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B. the power is equal to the resistive force times the velocity

51

A ball is thrown vertically upwards and comes down again. Air resistance is negligible. Which of the following graphs shows how the gravitational potential energy EP varies with time t ?

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D

52

A pump extracts water from a well of depth h at a constant rate of R kgs–1. What is the power required to raise the water?
A.    R/gh
B.     Rgh
C.     Rg/h
D.     hg/R

B. The power required is equal to the rate times gravity times height

53

A vehicle is driven up a hill at constant speed. Which of the following best describes the energy changes involved?
     A.     Chemical energy is converted into gravitational potential energy.
     B.     Chemical energy is converted into gravitational potential energy, sound and thermal energy.
     C.     Gravitational potential energy is converted into chemical energy.
     D.     Gravitational potential energy is converted into chemical energy, sound and thermal energy.

B.     Chemical energy is converted into gravitational potential energy, sound and thermal energy.

54

Which of the following is a correct definition of work?
     A.     Product of force and distance
     B.     Product of force and distance moved in the direction of the force
     C.     Product of power and time
     D.     Product of force and displacement

B.     Product of force and distance moved in the direction of the force

55

A satellite is in orbit about Earth. The satellite moves to an orbit closer to Earth. Which of the following correctly gives the change in the potential energy and the kinetic energy of the satellite?

               change in potential energy               change in kinetic energy

A.            decreases                                           increases

B.            decreases                                           decreases

C.            increases                                             increases

D.            increases                                            decreases

A. the potential energy energy decreases as the satellite moves closer, but the kinetic enrgy increases

56

A lift (elevator) is operated by an electric motor. It moves between the 10th floor and the 2nd floor at a constant speed. One main energy transformation during this journey is
     A.     gravitational potential energy → kinetic energy.
     B.     electrical energy → kinetic energy.
     C.     kinetic energy → thermal energy.
     D.     electrical energy → thermal energy.

D.     electrical energy → thermal energy

57

A constant force acts on a mass that is initially at rest. Which of the following graphs best shows how the kinetic energy EK of the mass changes with the work W done on the mass? Friction is negligible.

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A. kinetic energy and work are directly proportional