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Work and Energy
1. The third floor of a house is 8.0 m above street level. How much work is needed to move a 150 kg
refrigerator to the third floor?
2. Sally applies a horizontal force of 462 N with a rope to drag a wooden crate across a floor with a constant
speed. The rope tied to the crate is pulled at an angle of 56°.
a. How much force is exerted by the rope on the crate?
b. What work is done by Sally if the crate is moved 24.5 m?
c. What work is done by the floor through force of friction between the floor and the crate?
3. Mike pulls a sled across level snow with a force of 225 N along a rope that is 35° above the horizontal. If
the sled moved a distance of 65.3 m, how much work did Mike do?
4. An 845 N sled is pulled a distance of 185 m. The task requires 1.20 x 104 J of work and is done by pulling
on a rope with a force of 125 N. At what angle is the rope held?
5. Karen has a mass of 57.0 kg and she rides the up escalator at Woodley Park Station of the Washington D.C.
Metro. Karen rode a distance of 65 m, the longest escalator in the free world. How much work did the
escalator do on Karen if it has an inclination of 30°.
6. Brutus, a champion weightlifter, raises 240 kg a distance of 2.35 m.
a. How much work is done by Brutus lifting the weights?
b. How much work is done holding the weights above his head?
c. How much work is done lowering them back to the ground?
d. Does Brutus do work if the weights are let go and fall back to the ground?
e. If Brutus completes the life in 2.5 s, how much power is developed?
7. A force of 300 N is used to push a 145-kg mass 30 m horizontally in 3.00 s?
a. Calculate the work done on the mass?
b. Calculate the power?
8. Wayne pulls a 305 N sled along a snowy path using a rope that makes a 45° angle with the ground. Wayne
pulls with a force of 42.3 N. The sled moves 16 m in 3.0 s. What is Wayne’s power?
9. A car us driven at a constant speed if 21 m/s (76 km/h) down a road. The cars engine delivers 48 kW of
power. Calculate the average force of friction that is resisting the motion of the car.
10. What work is required to lift a 215 kg mass a distance of 5.65 m using a machine that is 72.5% efficient?
Conservation of Energy
1. A 98 N sack of grain is hoisted to a storage room 50 m above the ground floor of a grain elevator.
a. How much work was required?
b. What is the potential energy of the sack of grain at this height?
c. The rope being used to lift the sack of grain breaks just as the sack reaches the storage room. What
kinetic energy does the sack have just before it strikes the ground floor?
2. A 20 kg rock is on the edge if a 100 m cliff.
a. What potential energy does the rock possess relative to the base of the cliff?
b. The rock falls from the cliff. What is the kinetic energy just before it strikes the ground?
c. What speed does the rock have as it strikes the ground?
3. An archer puts a .30 kg arrow to the bowstring. An average force of 201 N is exerted to draw the string
back 1.3 m.
a. Assuming no frictional loss, with what speed does the arrow leave the bow?
b. If the arrow is shot straight up, how high does it rise?
4. A physics book, mass unknown, is dropped 4.5 m. What speed does the book have just before it hits the
ground?
5. A railroad car with a mass of 5.0 x 105 kg collides with a stationary railroad car of equal mass. After the
collision, the two cars lock together and move off at 4.0 m/s.
a. Before the collision, the first railroad car was moving at 8.0 m/s. What was its momentum?
b. What is the total momentum of the two cars after the collision?
c. Find the kinetic energies of the two cars before and after the collision.
d. Account for the loss of kinetic energy.
6. Shawn and his bike have a total mass of 45.0 kg. Shawn rides his bike 1.80 km in 10.0 min at a constant
velocity. What is Shawn’s kinetic energy?
7. Sally and Lisa have a mass of 45 kg each and they are moving together with a speed of 10 m/s.
a. What is their combined kinetic energy?
b. What is the ratio of their combined mass to Sally’s mass?
c. What is the ratio of their combined kinetic energy to Sally’s kinetic energy? Explain.
8. A 14,700 N car is traveling at 25 m/s. The brakes are suddenly applied and the car slides to a stop. The
average braking force between the tires and the road is 7100 N. How far will the car slide once the brakes
are applied?
9. A 15 kg cart is moving with a velocity of 7.5 m/s down a level hallway. A constant force of 10.0 N acts on
the cart and its velocity becomes 3.20 m/s.
a. What is the change in kinetic energy of the cart?
b. How much work was done on the cart?
c. How far did the cart move while the force acted?
10. Mary weighs 500 N and she walks down a flight of stairs to a level of 5.5 m below her starting point. What
is the change in Mary’s potential energy?
11. Ace raised a 12.0 N physics book from a table 75 cm above the floor to a shelf, 2.15 m above the floor.
What was the change in potential energy?