When a conservative force does work, you can get that energy changes forms, whether a conservative force does work or non-conservative does work. But when a conservative force does work, it’s easy to get the energy out of the system. And when a non-conservative force does work, it’s much tougher to get the energy out of the system.
To determine whether an object has conservative force or non-conservative forces:
1. The work done by a conservative force when displacing an object from point “a” to point “b” is independent of the path taken.
· Friction is a non-conservative force.
2. The work done by a conservative force over a closed path is always zero.
Example, in the problem of free fall (neglecting air drag) we have already studied, an object is thrown upward at a certain initial speed. It reaches a maximum height where its velocity is zero and then starts to descend with negative velocity. Clearly the body has lost all its kinetic energy once it has reached the maximum height, because it has zero velocity. However, as the objects falls back, when it reaches the ground again its speed is identical to its initial speed. What happened is that on the way up the force of gravity does a negative work on the object, while on the way down it does a positive work, giving back to the object the same energy it had taken away. Forces like this are called conservative, because their work is never wasted, it can always be recovered. Forces that are not conservative, whose work cannot be recovered, are called non-conservative. The best examples are friction and air drag. Non-conservative forces tend to disperse their work into forms that cannot be recovered by simply reversing the motion, such as heat and sound.
It is useful to recast the work-energy theorem in a new way based on the distinction of conservative and non-conservative forces. The theorem was: W = ΔKE
Now we can rewrite this as:
WNC + WC = ΔKE
because the net work is the sum of the work of conservative and non-conservative forces.
Understanding Conservative Forces
· The object regains initial motion (kinetic energy) on return to initial position in a closed path motion.
· Conservative force transfers energy "to" and "from" an object during a closed path motion in equal measure.
· Conservative force transfers energy between kinetic energy of the object in motion and the potential energy of the system interacting with the object.
· Work done by conservative force is equal to work done by it on reversal of motion.
· Total work done by conservative force in a closed path motion is zero.
Understanding Non-conservative Forces
· The speed and kinetic energy of the object on return to initial position are lesser than initial values in a closed path motion.
· Non - conservative force does not transfers energy "from" the system "to" the object in motion.
· Non - conservative force transfers energy between kinetic energy of the object in motion and the system via energy forms other than potential energy.
· Total work done by non-conservative force in a closed path motion is not zero.
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