[jyl]

Help me understand a physics concept. I don't get the intuition behind it.

It has to do with power.

I'll ask my question in the context of a pulley problem.

A block of mass M1 is on a frictionless plane, inclined at angle Z. A string is fastened to that block. The string passes over a frictionless pulley. The other end of the string is fastened to a hanging block of mass M2.

The force of gravity on block 2 is F2 = M2 x g. The component of the force of gravity on block 1 that is tangent to the plane is F1 = M1 x g x sin Z.

If M2 > M1 x sin Z, then block 2 will descend and pull block 1 up the inclined plane. The blocks will accelerate at A = [ (M2 - M1 * sin Z) / (M1 + M2) ] * g although mathematically block 1 has acceleration A and block 2 has acceleration -A. The blocks will have the same scalar component of velocity V.

Assuming the blocks start at rest, at time T, velocity V = A * T.

(Please correct me if I have it wrong so far. This is straight out of first year physics but college was a long time ago.)

I'm trying to calculate the power that block 2 applies to block 1. Block 2 is applying a constant force F2 = M2 * g to block 1. Power = force x velocity. So Power P = F2 x V.

(Still right?)

Now, this is what I don't intuitively get. As T increases, with constant A, velocity increases. That means power P increases. At T = 0. P = 0. The faster the masses move, the higher the power that is being applied to block 1.

If the force remains constant, how can power increase?

I don't understand this intuitively.

Actually, I always have trouble intuitively understanding power. I recall vaguely that in physics class, we always calculated Work, and then divided by time to get Power. We didn't start by calculating Power. Maybe that's why I never really got comfortable with Power.

Using that approach, I can see that block 2's kinetic energy increases at a doubly accelerating rate, so that in the 1 second interval from T = 1 to 2 there is a small increase in KE, while in the 1 second interval from T = 10 to 11 there is a much larger increase in KE. So if P = KE / T, the power in the first interval is small while the power in the second interval is much larger. But I don't "get it".