# One 'stupid' doubt

1. Aug 11, 2007

### menager31

We've got some small mass on the string which is in rotatory movement in horizontal plane. let,s say that the velocity is v and the radius is r. Let's increase the speed of the mass by 'doing some movements with our hand'( we hold the string).

Now the potential energy is greater than it was before. So the increment of potential energy is from the decrease of the kinematic energy? Or it's from the work which was made by our hand?

2. Aug 11, 2007

### HallsofIvy

Why do you say the potential energy is greater? If you increase the speed of the mass by 'doing some movements with our hand' then, since you have increased the speed, you have increased the kinetic energy, not the potential energy. And, yes, that increase is from the work done by your hand.

3. Aug 11, 2007

### menager31

ok, and what if the mass has got a charge and is moving in vertical magnetic field? the tension of the string increases and the mass goes higher. From what is the increment of the potential energy?(from the kinematic!?)