Name Period Date Chapter 8 nergy Work and Power … · Chapter 8 nergy Work and Power ... A car is...

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Chapter 8 Energy, Work, and Power

Doc Fizzix Practice Problems • Unit I Mechanics

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Example Problems

8.3 PowerE1. A monkey goes rock climbing. After 30 minutes the monkey has done 5900 J of

work scaling up a cliff, calculate the power out of the monkey.

Record all givens, draw a picture, arrow all vectors, write the formula, substitute and solve

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unitsa) E2. A monkey lifts a bucket of water 30 meters out of a well at a constant velocity. If the

bucket weighs 98 N and it takes 45 seconds to reach the top, calculate the work done by the monkey and the power output.


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E3. A 51 kg monkey climbs a 6 meter tall tree in 10 seconds, how much work does the monkey do and what is the power output of the monkey?


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unitsa) unitsb) E4. A monkey, while running, puts out 50 watts of power, calculate how much work the

monkey does running for 1.0 hour.


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E5. An 8 kg mass is lifted from the floor to the ceiling in a science room using a small electric motor that puts out 25 watts of power. If it takes 7 seconds to lift the mass to the ceiling, calculate the work done on the mass and the height of the science room.


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unitsa) unitsb) E6. A monkey has bought a piano but lives on the sixth-story of an apartment building

and needs to get the piano from the street 15 meters up to his flat. The piano will not fit through the doors into the apartment but will fit through the outside window. The monkey attaches a rope to the piano and the other end to a 1,750 watts motor attached to the top of the building. How much energy will it take to lift the 285 kg piano to the monkey’s flat and how long will it take?


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E7. A monkey is trying out his new Porsche. The monkey wants to “feel” the vehicles acceleration. Starting from rest the monkey pushes the gas pedal to the floor and the car goes from 0-60 mph (0-26.82 m/s) in 3.4 seconds. If the mass of the car is 1,361-kg calculate the change in kinetic energy and the power output of the Porsche.


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unitsa) unitsb) E8. A car is traveling on a level road at 16.67 m/s, the engine generates 12 hp (8948.4

watt). Determine the average retarding force due to wind resistance and friction.


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Student Problems

8.3 Power1. A monkey uses a horizontally force of 300.0 N to push a crate 30.0 meters in 3.00 sec.

Calculate the work done on the crate and the power output of the monkey.


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unitsa) unitsb) 2. A monkey pushes a wheelbarrow by exerting a 145 N force horizontally. The monkey

moves the wheelbarrow 60.0 meters at a constant speed for 25.0 sec. Calculate the work done and the power output by the monkey.


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3. A gorilla, a champion weight-lifting, raises 240 kg of weights a distance of 2.35 meters in 2.5 seconds. Calculate the work done lifting the weight and the power output.


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unitsa) unitsb) 4. A load of bricks, mass is 420 kg, is to be lifted by a winch to a height of 120 meters

taking 5.0 minutes. Calculate the minimum power of the winch motor.


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5. A motor puts out 120 watts of power for 5.0 seconds to raise a 15 newton object. Calculate the vertical distance the object was raised.


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unitsa) 6. A monkey uses a small 1.5 watt electric motor to lift a 0.50 kilogram banana at

constant speed. Calculate the vertical distance the banana can be lifted in 5.0 seconds.


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7. A monkey puts out 25 watts of power pushing horizontally on a crate with a force of 100 N in order to move the crate a distance of 15 meters. Calculate the work done by the monkey and the time it takes to do the work.


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unitsa) unitsb) 8. A manually operated winch is used to lift a 200 kg mass to the roof of a 10 meters

tall building. Assuming that you can work at a steady rate of 200 W, how long will it take you to lift the object to the roof?


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9. A car of mass 1550 kg can reach a speed of 60 mph (26.8 m/s) in 7.1 s. Calculate the change in kinetic energy and the average power needed to accomplish this.


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unitsa) unitsb) 10. A shot-putter accelerates a 7.30-kg shot from rest to 14.0 m/s. If this motion takes

2.00 seconds, what average power was supplied?


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11. An electric motor develops 65,000 W of power as it lifts a loaded elevator 17.5 meters in 35 seconds. How much force does the motor exert?


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unitsa) 12. It takes 10 seconds to lift a box 20.0 meters vertically using a cable attached to a

motor. If the motor generates 1150 watts of power, calculate the weight of the box.


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Monkey Busters

8.3 Power13. A car of mass 1214.5 kg is moving at a speed of 62.5 mph when it misses a curve in

the road and hits a bridge piling. If the car comes to rest in 0.236 seconds, how much average power (in watts) is expended in this interval?


unitsa) 14. An engine expends 40.0 hp in moving a car along a level track at a speed of 15.0 m/s.

How large is the total force acting on the car in the direction opposite to the motion of the car?


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15. A car traveling along a flat road at 100 km/h dissipates energy at the rate of 3 kW. What is the retarding force due to all frictional effects?


unitsa) 16. A 1500-kg car (including the mass of the driver) has an engine that provides 40 kW

at the drive shaft. If the car starts from rest, what is its velocity after 6 s?


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17. A car of mass 942.4 kg accelerates from rest with a constant power output of 140.5 hp. Neglecting air resistance, what is the speed of the car after 4.55 s?


unitsa) 18. A 20-hp motor is used to lift an elevator whose mass, when empty, is 1200 kg. If

this elevator should rise at 1 m/s what is the maximum additional load that can be placed in it, if all friction losses are neglected?


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19. An 1800-kg car is equipped with a 40-hp engine. Neglecting all friction losses, determine the maximum speed at which this car can climb a 15° grade.


unitsa) 20. A bicyclist coasts down a 7.0° slope at a steady speed of 5.0 m/s. Assuming a total

mass of 75 kg (bicycle plus rider), what must the cyclist’s power output be to pedal up the same slope at the same speed?


unitsa)

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