Sem I Final Review: Forces, Work, Energy and Power
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IB PHYSICS SL GOHS
SEM I FINAL REVIEW: FORCES, WORK, ENERGY AND POWER
1. A block is at rest on a rough plane inclined at an angle θ relatively to the horizontal.
F
The angle θ is slowly reduced. Which one of the following correctly describes the changes, if any, in the frictional force F and the coefficient of static friction between the block and the plane?
Frictional force F Coefficient of static friction
A. decreases increases
B. decreases constant
C. increases increases
D. increases constant (1)
2. A powered spaceship is moving directly away from a planet as shown below.
P
planet Spaceship
At point P the motors of the spaceship are switched off but the spaceship remains under the influence of the planet. Which one of the following graphs best represents the variation with time t of the velocity v of the spaceship after it passes point P?
A. v B. v
0 0 0 t 0 t
C. v D. v
0 0 0 t 0 t
1 (1)
2 3. The weight of a mass is measured on Earth using a spring balance and a lever balance, as shown below.
spring balance lever balance
What change, if any, would occur in the measurements if they were repeated on the Moon’s surface?
Spring balance Lever balance
A. same same
B. same decrease
C. decrease same
D. decrease decrease (1)
4. A block is placed on a horizontal rough surface. A horizontal force F is applied to the block, as shown below.
block force F
The force required to keep the block moving at constant speed is less than the force required to make the block move from rest. The explanation for this observation is that
A. before the block moves, the force F must also produce a turning moment.
B. a force is not required to keep the block moving at constant speed.
C. friction has to be overcome to make the block move.
D. the maximum static friction forces are greater than the maximum dynamic friction forces. (1)
3 5. The displacement d of a particle in a wave varies with distance x along a wave and with time t as shown below.
d d
0 0 0 l l 3l 2l x 0 2 3 4 t 2 2
Which expression gives the speed of the wave?
l A. 4
l B. 2
l C.
2l D. (1)
6. An object is taken from the Earth to the Moon. What change, if any, occurs in its gravitational mass and in its inertial mass?
Gravitational mass Inertial mass
A. decreases decreases
B. decreases unchanged
C. unchanged decreases
D. unchanged unchanged (1)
4 7. A block rests on a rough horizontal plane and a force P is applied to the block as shown.
N
P
The normal reaction between the plane and the block is N and the frictional force between the block and the plane is F. The coefficient of static friction between the block and the plane is μS and initially P is zero.
As P is increased in value, which one of the following statements is true concerning the relationship between F, N and μS?
A. F is always equal to μSN.
B. F is always greater than μSN.
C. F is always less than μSN.
D. F can be equal to μSN. (1)
8. A bird of weight W lands at the midpoint of a horizontal wire stretched between two poles. The magnitude of the force exerted by each pole on the wire is F.
F F
W
The bird will be in equilibrium if
A. 2F > W.
B. 2F = W.
C. 2F < W.
D. F = W. (1)
5 9. A metal disc is acted upon by a number of forces. The forces are all in the plane of the disc and the weight of the disc is negligible.
In which of the following situations is the disc in equilibrium?
A. B. F F F F 3F 2F F
2F C. D. F F F 2F 2F F (1)
10. A mass is suspended from the roof of a lift (elevator) by means of a spring balance, as illustrated below.
lift (elevator)
mass
The lift (elevator) is moving upwards and the readings of the spring balance are noted as follows.
Accelerating: Ra
Constant speed: Rc
Slowing down: Rs
6 Which one of the following is a correct relationship between the readings?
A. Ra > Rc
B. Ra = Rs
C. Rc = Rs
D. Rc < Rs (1)
11. A friction force f is acting on a block of weight w sliding down an incline at a constant speed. The force N is the normal reaction of the incline on the block. Which of the following free-body diagrams best represents the forces acting on the block?
A. f B. N N f
w w
C. f D. N N
f
w w (1)
12. A block is sliding down a rough slope. The force of sliding friction between the block and the slope depends on
A. the speed of the block.
B. the surface area of the block.
C. the normal reaction between the block and the slope.
D. the acceleration of the block. (1)
7 13. Which one of the following statements correctly defines the gravitational potential at a point P in a gravitational field?
A. The work done per unit mass in moving a small mass from point P to infinity.
B. The work done per unit mass in moving a small mass from infinity to point P.
C. The work done in moving a small mass from infinity to point P.
D. The work done in moving a small mass from point P to infinity. (1)
14. 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 (1)
15. An object of mass m1 has a kinetic energy K1. Another object of mass m2 has a kinetic energy
K1 K2. If the momentum of both objects is the same, the ratio is equal to K2
m A. 2 . m1
m B. 1 . m2
m C. 2 . m1
m D. 1 . m2 (1)
8 16. The graph below shows the variation with displacement d of the force F applied by a spring on a cart.
5
4
3 F / N 2
1
0 0 1 2 3 d / 10–2 m
The work done by the force in moving the cart through a distance of 2 cm is
A. 10 × 10–2J.
B. 7×10–2J.
C. 5×10–2J.
D. 2.5×10–2J. (1)
17. The diagram below shows the variation with displacement x of the force F acting on an object in the direction of the displacement.
F
R
Q S P
0 W V T 0 x1 x2 x
9 Which area represents the work done by the force when the displacement changes from x1 to x2?
A. QRS
B. WPRT
C. WPQV
D. VQRT (1)
18. 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 Efficiency
A. E = W + H W
W B. E = W + H E W C. E + H = W H W D. E + H = W E – H (1)
19. A machine lifts an object of weight 1.5 × 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 × 103 J.
B. 1.2 × 104 J.
C. 1.8 × 104 J.
D. 7.5 × 104 J. (1)
10 20. An electric train develops a power of 1.0 MW when travelling at a constant speed of 50 ms–1. The net resistive force acting on the train is
A. 50 MN.
B. 200 kN.
C. 20 kN.
D. 200 N. (1)
21. A stone of mass m is attached to a string and moves round in a horizontal circle of radius R at constant speed V. The work done by the pull of the string on the stone in one complete revolution is
A. zero.
B. 2πmV2.
2πmV 2 C. . R
2πmV D. . R (1)
22. The variation with time of the vertical speed of a ball falling in air is shown below.
Speed
0 0 T time
11 During the time from 0 to T, the ball gains kinetic energy and loses gravitational potential energy ΔEp. Which of the following statements is true?
A. ΔEp is equal to the gain in kinetic energy.
B. ΔEp is greater than the gain in kinetic energy.
C. ΔEp is equal to the work done against air resistance.
D. ΔEp is less than the work done against air resistance. (1)
23. Which of the following quantities are conserved in an inelastic collision in an isolated system of two objects?
Linear momentum of system Kinetic energy of system
A. Yes Yes
B. Yes No
C. No Yes
D. No No (1)
24. The diagram below represents energy transfers in an engine.
input energy useful output energy engine EIN EOUT
wasted energy EW
12 The efficiency of the engine is given by the expression
E A. W . EIN
E B. W . EOUT
E C. OUT . EIN
E D. OUT . EW (1)
25. Which of the following involves a change in the total energy of the objects?
A. Some ice and water as the ice melts at constant temperature.
B. An electron accelerated by a magnetic field.
C. A satellite in a circular orbit round the Earth.
D. A stone falling in a vacuum towards the Earth’s surface. (1)
26. An amount Q of energy is supplied to a machine. The machine does useful work W and an amount R of energy is wasted, as illustrated below.
energy supplied Q useful work W machine
wasted energy R
13 Which one of the following is a correct expression for the efficiency of the machine?
W A. Q
R B. Q
W R C. Q
W R D. Q (1)
27. 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.
1 C. mv2 2
D. fs. (1)
28. 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.
F
d
Which one of the following is the correct expression for the work done by the force?
A. Fd
B. Fd sinθ
C. Fd cosθ
D. Fd tanθ (1)
14 29. 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 %. (1)
ANSWERS!!!!!!!
1. B [1]
2. C+D [1]
3. C [1]
4. D [1]
5. B [1]
6. D [1]
7. D [1]
8. A [1]
9. C [1]
10. A [1]
11. B [1]
12. C [1]
13. B [1]
14. A
15 [1]
15. A [1]
16. C [1]
17. D [1]
18. B [1]
19. D [1]
20. C [1]
21. A [1]
22. B [1]
23. B [1]
24. C [1]
25. A [1]
26. A [1]
27. D [1]
28. C [1]
29. D [1]
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