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Work
A force does work on a body when it causes a displacement.
There is no work done by a force if it causes no displacement.
Forces perpendicular to displacement, such as the normal force, can do no work.
For example, centripetal forces never do work.
Calculating Work
W = F • s = F s cos 
W =  F(x) dx
W =  F • ds
SI Unit: Joule (N m)
The area under the curve of a graph of force vs displacement gives the work done by the force.
Problem: Work (CM 1993)
14. A weight lifter lifts a mass m at constant speed to a height h in time t How much work is done by the weight lifter?
(A) mg
(B) mh
(C) mgh
(D) mght
(E) mgh/t
Show your work:
Kinetic Energy (K)
A form of mechanical energy
Energy due to motion
K = ½ m v2
K: Kinetic Energy in Joules.
m: mass in kg
v: speed in m/s
Problem: Kinetic Energy (CM 1993)
6.
A ball is thrown upward. At a height of 10 meters above the ground, the ball has a potential energy of 50 joules (with
the potential energy equal to zero at ground level) and is moving upward with a kinetic energy of 50 joules. Air friction
is negligible. The maximum height reached by the ball is most nearly
(A) 10 m
(B) 20 m
(C) 30 m
(D) 40 m
(E) 50 m
Show your work:
Net Work
Net work (Wnet) is the sum of the work done on an object by all forces acting upon the object.
Wnet = W
The Work-Energy Theorem
Wnet = KE
When net work due to all forces acting upon an object is positive, the kinetic energy of the object will increase.
When net work due to all forces acting upon an object is negative, the kinetic energy of the object will decrease.
When there is no net work acting upon an object, the kinetic energy of the object will be unchanged.
Kinetic Energy (K)
A form of mechanical energy
Energy due to motion
K = ½ m v2
K: Kinetic Energy in Joules.
m: mass in kg
v: speed in m/s
Problem: Work-Energy Theorem (CM 1984)
15.
The following graphs, all drawn to the same scale, represent the net force F as a function of displacement x for an
object that moves along a straight line. Which graph represents the force that will cause the greatest change in the kinetic
energy of the object from x = 0 to x = x1?
State your reasoning:
Problem: Work-Energy Theorem (CM 1988)
17. A rock is lifted for a certain time by a force F that is greater in magnitude than the rock's weight W. The change in
kinetic energy of the rock during this time is equal to the
(A) work done by the net force (F - W)
(B) work done by F alone
(C) work done by W alone
(D) difference in the momentum of the rock before and after this time
(E) difference in the potential energy of the rock before and after this time.