General physics question: can fricton work be equal to kinetic energy, but opposite?

Question

I'm study for AP Physics C exam...

I'm usin' Princeton Review AP Physics B & C...

Appreciate if we can work together and prepare together for comin' AP exam!

Peacee

Answer 1

What do you mean by opposite? Work and energy are scalars so there is no opposite (no directional dependence). Work done by friction and change in kinetic energy can most definitely be equal if no other forces are present on an object.

For example, if you are driving your car down a straight, flat road and you put it in neutral, your car will slow down because of work being done by friction between the pavement and your tires (of course ignoring air resistance). This work done by friction is equivalent to the change in kinetic energy of your car. This must be true in this situation for energy to be conserved.

Answer 2

Work done by friction is almost always negative. This is because friction does work against the motion. I don't mean the term against as in the opposite direction... rather, it means that it is removing energy from the system.

Any non-conservative force behaves in this way... it either adds energy into the system (for example, you pushing your little brother on a swing) or removes energy from the system (heat due to friction, air resistance, sound...)

The work done by non-conservative forces is equal to the change in energy.

Wnc = Ef - Ei

If you can learn what that statement means, you will be able to do ANY energy problem you are faced with!

Answer 3

Any equality between kinetic energy and friction would involve not only the mechanical work but also the heat energy created by friction and lost to the environment.