# Instaneous power delivered by the force at any timet

1. Oct 16, 2004

### hauthuong

A force F acts on a particle of mass m. The particle starts from rest at t=0.
a) show that the instaneous power delivered by the force at any timet is (F^2/m)*t
Please give some directions how to solve it . Thank you

2. Oct 16, 2004

### Parth Dave

Any idea where to start? (hint: what is the formula for power)

3. Oct 16, 2004

### hauthuong

formula P=W/t or P=FV , and I don't know how to get t and and m involve in the formula

4. Oct 17, 2004

### BLaH!

Think of the definition of velocity. $$v = a t$$. And then use Newton's Seond Law for $$a$$.

5. Oct 17, 2004

### hauthuong

Thank you very much, I got it, and I have another homework.
A raindrop (m=3.35*10^-5 kg) falls vertically at a constant speed under the influence of gravity and air resistance. after the drop has fallen 100m,
a)what is the work done by gravity?
b)the energy discipated by air resistance?
I got no clue for air resistance . Thank you

6. Oct 17, 2004

### MiGUi

If the raindrop where in the vacuum, the falling speed will not be constant. But when we do the experiment in a fluid, then a viscosity force appears and makes the raindrop or something else reach a constant speed. This force is proportional to the speed, so you may use newtonian physics...

7. Oct 17, 2004

### hauthuong

is it called resistive force?