Energy 2010
1.
Together, two students exert a force of 825 N in pushing a car a distance of 40 m.
(a) How much work do the students do on the car?
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(b) If the force was doubled, how much work would they do pushing the car the same distance?
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2.
An electric motor develops 69 kW of power as it lifts a loaded elevator 17.5 m in 35 s. How much force does the motor exert?
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3.
A 0.250 kg ball falls 2.6 m. How much work does the force of gravity do on the ball?
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4.
Diego pulls a 4.5 kg sled across level snow with a force, F, of 215 N on a rope that is 35.0° above the horizontal, as shown in Figure 10-18. If the sled moves a distance of 62.8 m, how much work does Diego do?
5.
Maricruz slides a 50 kg crate, m, up an inclined ramp 2.0 m long and attached to a platform 1.0 m above floor level, as shown in Figure 10-19. A 385 N force, F, parallel to the ramp, is needed to slide the crate up the ramp at a constant speed.
(a) How much work does Maricruz do in sliding the crate up the ramp?
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(b) How much work would be done if Maricruz simply lifted the crate straight up from the floor to the platform?
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6.
In Figure 10-20, the magnitude of the force necessary to stretch a spring is plotted against the distance the spring is stretched.
Figure 10-20
(a) Calculate the slope of the graph, k, and show that F = kd, where k = 25 N/m.
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(b) Find the amount of work done in stretching the spring from 0.00 m to 0.20 m by calculating the area under the graph from 0.00 m to 0.20 m.
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(c) Show that the answer to part b can be calculated using the formula W = 1/2 kd2, where W is the work, k = 25 N/m (the slope of the graph), and d is the distance the spring is stretched (0.20 m). (Do this on paper. Your instructor may ask you to turn in this work.)
Key: paper submission
7.
The graph in Figure 10-22 shows the force and displacement of an object being pulled.
(a) Calculate the work done to pull the object 8.0 m.
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(b) Calculate the power that would be developed if the work was done in 1.8 s.
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8.
A 18 kg rock, m, is on the edge of a 80 m cliff, h, as shown in Figure 11-20. Assume that air resistance is negligible.
(a) What potential energy does the rock possess relative to the base of the cliff?
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(b) The rock falls from the cliff. What is its kinetic energy just before it strikes the ground?
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(c) What speed does the rock have as it strikes the ground?
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9.
An archer puts a 0.27 kg arrow to the bowstring. An average force of 211 N is exerted to draw the string back 1.3 m. Assume that air resistance is negligible.
(a) Assuming that all the energy goes into the arrow, with what speed does the arrow leave the bow?
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(b) If the arrow is shot straight up, how high does it rise?
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10.
Lorena's mass is 27 kg. She climbs the 4.8 m ladder of a slide and reaches a velocity of 3.4 m/s at the bottom of the slide. How much work was done by friction on Lorena? Assume that air resistance is negligible.
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11.
A 14680 N car, m, is traveling at 25 m/s. The brakes are applied suddenly, and the car slides to a stop, as shown in Figure 11-17. The average braking force between the tires and the road is 7140 N. How far will the car slide once the brakes are applied? Assume that air resistance is negligible.
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12.
A physics book of unknown mass is dropped 4.45 m. What speed does the book have just before it hits the ground? Assume that air resistance is negligible.
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