friction and power

1. Because when we are calculating power, we are intended to measure the "action" power. If we take the reaction into account, it will result in the conservation of energy, always zero balance.

2. Because the extra force is used to overcome the friction, but not to move the object. The more force you use, the faster the object moves, but the work done is also the same (because the energy goes to speed).


2009-03-12 23:26:39 補充：
power = work done / time = force * displacement / time = force * velocity.

Gain in PE will normally result in loss of KE (mv^2). Velocity can reflect the state of PE and thus PE is not miss out. If we take both PE and KE, it will always give a "zero" sum (no change in energy => no power).

2009-03-20 01:40:34 補充：
you can forget about the speed.