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Physics
Work
Name ___________________________
Work is defined as the force on an object in the direction of an object’s motion multiplied by the idstance
the object moves. Or: W=(F)(d). If the force is at an angle to the object’s motion, only the component of
the force that is parallel to the object’s motion is used to determine the work on the object.
Work is defined as negative when the force is in the direction opposite the displacement. Friction forces
always do negative work on an object. When things move against gravity, gravity is said to do negative
work on the object.
1a) Determine the work a hiker must do on a 15.0 kg backpack to carry it up a hill 30⁰ to the horizontal at
constant velocity. Assume the height of the hill is 10.0m above the level ground.
b) Determine the work done by gravity on the backpack.
c) Determine the net work done on the backpack.
d) Redo part a, above, if the hiker climbs the same hill from a different side where the angle to the
horizontal is 45⁰.
2) A 75 kg firefighter climbs a flight of stairs 10.0m high. How much work is required by the
firefighter if he climbs at constant speed?
3) A 900 N crate rests on the floor. How much work is required to move it at constant speed
a) 6.0 m along the floor against a friction force of 180 N?
b) 6.0 m vertically?
4) How much work does a mover do while (horizontally) pushing a 150 kg crate 12.3 m across a rough
floor without acceleration, if the coefficient of kinetic friction is 0.70?
5) A car does 7.0 x 104 J of work in traveling 2.8 km at constant speed. What was the average retarding
force (from road friction and air resistance combined) acting on the car?
6) How high will a 0.325 kg rock go if thrown straight up by someone who does 115 J of work on it?
Neglect air resistance.
7) Eight books, each 4.6 cm thick with mass 1.8 kg, lie flat on a table. How much work is required to stack
them one on top of another?
8) What is the minimum work needed to push a 1000 kg car 300m up a 17.5⁰ incline?
a) ignore friction
b) Assume the coefficient of friction is 0.25.