Download Handout Topic 2 Work , Energy, Power, Efficiency

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Review Handout “ Some Review Questions” :
Work, Energy , Power and Efficiency
1. The variation with time of the vertical speed of a ball falling in air is shown below.
Speed
0
0
T
time
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.
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2. 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
–2
d / 10 m
3
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.
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3. 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
0
V
x1
T
x2
x
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
4. 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%.
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5. This question is about work, energy and power.
(a)
Define the work done by a force.
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A body of mass m is in a gravitational field of strength g. The body is moved through a
distance h at constant speed v in the opposite direction to the field.
(b)
Derive an expression in terms of
(i)
m, g and h, for the work done on the body;
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(c)
A mass falls near the Earth’s surface at constant speed in still air. Discuss the energy
changes, if any, that occur in the gravitational potential energy and in the kinetic
energy of the mass.
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6. Tsokos # 11 p. 113
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