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Practice A, Work, pg 162
[1] 1.50 x 107 J [2] 700
J [3] 1.6 x 103 [4]
11.1 m
Section 5.1 Review Q, pg 163:
1a] negative [b] positive [c] negative
2] Your Work = Force x Distance
Neighbor Work = 1/2 Force x 4 Distance = 2 Force x Distance
Neighbor does twice your work.
3a] 8280 J [b] -7920 J [c] 360 J
4] Work In = FxD = mgD = 0.075x9.81x1.32 = 0.97119 J--Sig Fig--> 0.97
Work Out = FrictionxD = 0.350x1.32 = 0.73575 J--Sig Fig-->0.736
Net Work = Work In - Work Out = 0.97 - 0.736 = 0.234 J
5a] everyday sense [b] everyday sense [c] scientific sense
6a] yes [b] no [c] yes
Ch 5 Review Q, Work, pg 184:
1] No, a change in speed corresponds to a change in kinetic energy, which cannot
occur without work (either positive or negative) being done on the object.
2a] yes, positive [b] no [c] yes, positive [d] yes, negative
3] No, force would decrease, but distance would increase, which would keep work
constant.
4] The tension is perpendicular to the bob’s motion, so it does not do work on
the bob. The component of the bob’s weight that is perpendicular to the bob’s
motion does not do work on the bob, but the component that is in the direction
of motion does.
5] The car leaving longer skid marks was moving faster.
Car #2 Work = ΔKE--->F2D = KEf - KEi---> F2D = 0 - mV22/2
Car #1 Work = ΔKE--->F2D = KEf - KEi---> FD = 0 - mV12/2
Car# 2 = 2 = V22 --> V2 = V1√2
= V1x1.41; V2 was moving 41% faster
Car #1 V12
6] yes; no; yes, the ball’s weight and air resistance.
7] +53 J, -53 J
8] 240,000 J
9] 47.5 J
10a] 6230 J [b] -6230 J [c] 0.64
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Practice B, Kinetic Energy, pg 166
1] 1.7x102 m/s [2]
38.8 m/s
3] the bullet with the greater mass; 2 to 1
4] 1/4 [5] 1.6 x 103
Practice C, Work-Kinetic Energy Theorem, pg 168:
1] 7.8 m [2] 21 m [3] 5.1 m [4] 300 N
Practice D, Potential Energy, pg 172:
1] 3.3 J [2] 0.031 J 3a] 785 J [b] 105 J [c] 0.00 J
Section 5.2 Review Q, pg 172:
1] 0.0044 J [2] 2.8 m/s [3] 0.0618 J
4a] kinetic energy
b] Nonmechanical energy
c] kinetic energy, gravitational potential energy
d] elastic potential energy
5] The heated water is an instance of Nonmechanical energy, because its mass is
not displaced with a velocity or with respect to a zero position, as would be
the case for various types of mechanical energy. The bicycle and football both
have masses in motion, so they have kinetic energy. The wound spring has been
displaced from its relaxed position and so has elastic potential energy, while
the football is above the ground and therefore has a gravitational potential
energy.
Ch 5 Review Q, Energy, pg 184:
11a] no [b] yes [c] no
12] No, kinetic energy cannot be negative because mass is always positive and
the speed term of the equation is squared.
13] Yes, because potential energy depends on the distance to an arbitrary zero
level, which can be above or below the object.
14] 1 to 25
15] The gravitational force does not do work on the satellite because the force
of gravity is always perpendicular to the path of the motion.
16] The work required to stop the car equals the car’s initial kinetic energy.
If speed is doubled, work is quadrupled. Thus, the car will travel 140 m. Its
kinetic energy is changed into heat energy by friction.
17] Work must be done against gravity in order to climb a staircase at a
constant speed. Walking on a horizontal surface does not require work to be
done against gravity.
18] The work done by friction equals the change in mechanical energy, so the
particle’s speed decreases.
19] 75625 J
20] 16583 m/s
21] 20 m
22] 1.4 m
23a] 5400 J, 0 J; 5400 J [b] 0 J, -5400 J; +5400 J
c] +2700 J, -2700 J; 5400 J
24a] -19.6 J [b] 39.2 J [c] 0 J
25a] 0.400 J [b] 0.225 J [c] 0 J
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Practice E, Conservation of Mechanical Energy, pg 177
1] 20.7 m/s [2] at 5m Vf = 9.9 m/s, at 0 m Vf = 14 m/s
3] 14.1 m/s [4] 0.25 m [5] 0.18 m
Section 5.3 Review Q, pg 178:
1] 2.93 m/s
2] No, the roller coaster will not reach the top of a second hill. If the total
mechanical energy is constant, the roller coaster will reach its initial height
and then begin rolling back down the hill.
3a] yes [b] no [c] yes, if air resistance id disregarded
4] The downward-sloping track converts potential energy to kinetic energy.
Levers employ kinetic energy to increase potential energy. Springs and elastic
membranes convert kinetic energy to elastic potential energy and back again.
Mechanical energy is not conserved; some energy is lost because of kinetic
friction.
Ch 5 Review Q, Conservation of Mechanical Energy, pg
186:
26a] Nonmechanical [b] Mechanical [c] Mechanical [d] Mechanical
e] both
27] As the athlete runs faster, KE increases. As he is lifted above the ground,
KE decreases, as PEg and PEelastic increase (PEelastic comes from the bent
pole). At the highest point, KE = 0 and Peg is at its maximum value. As the
athlete falls, KE increases and Peg decreases. When the athlete lands, KE is at
its maximum value and Peg = 0.
28] The ball will not hit the lecturer because, according to the principle of
energy conservation, it would need an input of energy to reach a height greater
than its initial height. If the ball were given a push, the lecturer would be
in danger.
29a] Athlete does work on the weight. Peg increases
b] No work done on the weight. Peg is constant
c] Athlete does negative work on weight. Peg decreases
30] at the ball’s lowest height; at its maximum height.
31] no, because energy wouldn’t be conserved.
32] two, gravitational potential energy and elastic potential energy;
yes, because total mechanical energy is conserved if there is no
dissipation of energy by friction.
33] 12.0 m/s
34a] 10.9 m/s [b] 11.6 m/s
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Practice F, Power, pg 181
1] 66 kW [2] 23800 W [3] 2.61 x 108
s (8.27 years)
4] 3600 s = 1 h [5a] 7.50 x 104
J [b] 2.50 x 104 W
Section 5.4 Review Q, pg 181:
1] 12.3 s, Wk = 2450 J
2] 613 W, 2450 J
3] Power equal energy transferred divided by time of transfer.
4] A powerful engine is capable of doing more work in a given time. The force
and speed delivered by a powerful engine is large relative to less powerful
engines.
Ch 5 Review Q, Power, pg 186:
35] 17.2 s [36] 5.9 x 108
W
37a] 0.633 J [b] 0.633 J [c] 2.43 m/s [d] 0.422
J, 0.211 J
38] 0.265 m/s
39] 5.0 m
40] 1200 J
41] 2.5 m
42] 10.2 m
43] Although the total distance traveled by each ball is different, the
displacements are the same, so the change in potential energy for each ball is
the same. Also, each ball has the same initial kinetic energy, so the final
kinetic energy of each ball (and thus the speed of each) will be the same.
44a] 1.2 J [b] 5.0 m/s [c] 6.3 J
45a] 61 J [b] -45 J [c] 0 J
46] 24000 J
47a] 28.0 m/s [b] 30.0 m above the ground
48a] 5.42 m/s [b] 0.300 [c] -147 J
49] 0.107
50a] 310 J [b] -150 J [c] 180 N
51a] 66 J [b] 2.3 m/s [c] 66 J [d] -16 J
52a] 1.45 m [b] 1.98 m/s [c] 5.33 m/s
1] D [2] H [3] C [4] F [5] D [6] J [7] B
[8] J [9] A [10] G
11] 206 W [12] V = √2gh [13] 4.4 m/s [14] 1200 J
15] 1200 J [16] 1900 J [17] 290 m
Q4 Note: KE = mV2/2, if m is measured in grams
KE is measured in millijoules (mJ).
Measure graph at t = 0 sec: 600 mJ = 28 mm
At t = 4.5 s, KE = 14 mm: KE/14 mm = 600 mJ/24 mm
---> KE = 14x600/24 = 350 mJ
KE = mV2/2 _________
350 mJ = 75V2/2---> V2 = 2x350/75---->V = √2x350/75) =
3.06 m/s
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