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Conservative and Nonconservative Forces
| Conservative Forces |
The work done against conservative forces
does not depend on the pathway taken to do the work. Gravity is a
conservative force. The same amount of work is required to lift a
mass 20.0 m or push it up an inclined plane to a height of 20.0 m if there
is no friction. The elastic force of a spring is also a
conservative force.
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| Nonconservative Forces |
The work done against nonconservative forces
does depend on the pathway taken to do the work. Friction is a
nonconservative force. The amount of work necessary to slide a box
across the room is different when pushed straight across to room compared
to following the walls of the room to get to the same point.
Remember, the force of friction is always directly against the motion of
the object. |
Potential Energy can only be given for conservative forces. Potential
Energy is energy dependent on position or configuration. It is NOT valid
if a position does not have a unique energy. We can not have a Potential
Energy of friction because the energy is dependent on the pathway.
W net = WC + W NC
When conservative
and nonconservative forces act on an object
W net = 1/2 mv22 - 1/2 mv12
= /\KE
Defined earlier
WC = PE1 - PE2
Defined earlier
Combining and rearranging equations
W NC = W net - Wc
W NC = /\KE + PE2 - PE1
W NC = /\KE + /\PE
This allows us to easily measure Nonconservative Work and Force.
All of the forces acting on a body must be included either in the
potential energy (if it is a conservative force) or in the work term, W NC,
but not in both. | 
