Work & Power Problems
Note: 1 horsepower = 745.7 W
1.
How much work is required to drag a 75.0 kg cart along the ground for
2.00 km?
(a) without acceleration if the coefficient of friction was 0.060?
(b) with an acceleration of 1.15 m/s2 with the same coefficient of
friction?
(c) How much work is done by friction in (a) and (b)?
2.
An 80.0 kg sled is pulled a distance of 175 m. The task requires 1.20 ×
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?
3.
4.
A 15.0 kg box is being pulled by John across the floor. John is exerting
a force of 145 N, and the coefficient of friction is 0.190. He pulls the
box for 12.0 s.
a) How much work was done by John?
b) How much work was done by friction?
c) Calculate the net work done on the box.
A rope is used to pull a metal box 25.0 m across the floor. The rope is
held at an angle of 36.0° with the floor and a force of 628 N is used.
How much work does the force on the rope do?
5.
A student picks up a 2.25 kg book from the floor to a height of 1.25 m.
He carries the book 6.85 m to the bookcase and places the book on a
shelf that is 0.350 m high. How much work does he do on the book?
6.
How far must a 250.0 kg pile driver fall if it is to be capable of doing
17500 J of work?
7.
Calculate the work done for the following.
(a) A person pushes a 6.00 kg box with a force of 50.0 N for 2.00 m.
(b) A person pushes a 6.00 kg box with a force of 50.0 N for 3.00 s at
a constant speed of 1.25 m/s.
(c) A person pushes a 6.00 kg box with a force of 50.0 N for 3.00 s at
an acceleration of 1.25 m/s 2.
8.
A father is pulling his two young children in their sled (total mass = 90.0
kg) with a force of 525 N for a distance of 28.0 m. Calculate the work
that would be done by the father in each of the following cases.
(a) The snow provides no friction.
(b) One of the children drags her hands in the snow, producing a
frictional force of 40.0 N.
(c) What visible difference would you see in the motion between a) and
b)?
9.
A person is pushing a refrigerator across a floor at a constant speed.
Is the person doing work? Is the refrigerator gaining energy? Explain.
10 A 575 kg mass is placed on a forklift that can generate 950 W of
power. What is the constant speed of the forklift while lifting this
load?
11. If a car generates 14.0 hp when traveling at a steady 75.0 km/h, what
must be the average force exerted on the car due to friction and air
resistance?
12. How long will it take a 1550 W motor to lift a 425 kg piano to a window
40.0 m above the ground?
13. A 1275 kg car slows down from 90.0 km/h to 60.0 km/h in about 7.00 s
on a level surface when it is in neutral. Approximately what power is
needed to keep the car traveling at a constant 80.0 km/h?
14. A shot-putter accelerates a 7.25 kg shot from rest to 40.0 km/h. If
this motion takes 2.25 s, what average power was developed?
15. How much work (in J) can a 5.00 hp motor do in 2.00 h?
16. A pump can lift 5.25 kg of water per minute through a height of 2.65 m.
What output rating (watts) should the pump motor have?
17. Water is pumped up to a water tower that is 85.0 m high. The flow rate
up to the top of the tower is 60.0 L/s. What power is required to keep
up this flow rate to the tower? (note: 1.00 L of water has a mass of
1.00 kg)