Chapter 7 Energy
Exercises
19. PE is max at the top and KE is max at the bottom of the swing. In between the top and bottom is where PE and KE are half.
27. Work is scalar and has no direction.
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Work = F(horizontal component) * distance. |
45. Air resistance may slow the ball on return and it would have less KE. The ball would give the air molecules energy, so it does comply with the Conservation of Energy.
46. Both would hit with the same speed, because each ball starts with the same KE and PE. The KE given to the ball going up changes to PE then comes back to the starting point as KE. The two balls would hit the ground with the same KE.
64. If the engine were 100% efficient:
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it would not be warm to touch, |
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it would not heat the air from the exhaust, |
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it would not make noise, |
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it would not vibrate, and |
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it would not have any unused fuel. |
68.
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If take in more energy than expend, then excess is stored chemical energy (fat). |
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If expend more energy than take in, then the body burns fat. |
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Undernourished person would breakdown muscle tissue to perform extra work (not last long). |
Problems
4. KE = ½ mv2 so
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if the speed is tripled, then |
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KE will be (X3)2 or |
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15 m X 9 = 135 m. |
7. WorkIN = WorkOUT
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ForceIN * d = ForceOUT * d |
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100 N * 10 cm = F * 1 cm |
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F = 1000 N |
8. W = Fg = m * g = 600 N is the force exerted by the skydiver.
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Power = Work/time = |
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Fg * d/time = 30,000 J/s = |
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30,000 W = 30 kW |
11. IF 25% efficieny of 40 MegaJoules/L = 10 MegaJoules/L, then
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Force * d = 500 N * d = 10 MegaJoules/L |
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So, D = 10 MegaJoules/L/500 N = |
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10,000,000 J/L/500 N = 20,000 m/L = 20 km/L |
12. Mechanical Efficiency = EnergyOUT/EnergyIN
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Inactive Person = 1 W/100 W = 1% |
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Cyclist = 100 W/1000 W = 10% |