# Homework Help: Force and momentum

1. Jan 9, 2005

### UrbanXrisis

If I dropped a 5kg ball, from a 2m distance, the force acting upon it will be constant right? How would I get the final velocity of the ball before it hit the floor using this equation:
$$\vec{F}=\frac{d\vec{p}}{dt}$$

2. Jan 9, 2005

### Gokul43201

Staff Emeritus
If you can find time taken, T , then rewriting your equation

$$\int _0 ^T\vec{F}dt =\vec{p}_{fin}-\vec{p}_{in}$$

3. Jan 9, 2005

### dextercioby

It is quite weird to chose this equation,which assumes many calculations which could be shortcutted by applying the law of total energy conservation.

Daniel.

PS.In physics problem,always try to chose the easiest path to reach at your results...It saves energy,paper and neurons...

4. Jan 9, 2005

### UrbanXrisis

It is needed that use that equation, I'm trying to prove newton's second law in a momentum project.

The force is constant so time is not dependant right?
F=pf-pi
ma=mv-0
a=v?

5. Jan 9, 2005

### HallsofIvy

Why in the world would you think F= pf- pi?? That is NOT the equation that you quote. The equation you quote is
$$\int _0 ^T\vec{F}dt =\vec{p}_{fin}-\vec{p}_{in}$$

The left side is $\int_0^T\vec{F}dt$, not F!

6. Jan 9, 2005

### dextercioby

This is absurd.Newton's laws are postulated,axioms,they cannot be proved.It's like proving Schroedinger' equation,or the postulates of thermodynamics or the postulates of statistical mechanics (the axiomatical approach)... :grumpy:

Daniel.

7. Jan 9, 2005

### UrbanXrisis

OKay, back to my example, I dropped a 5kg ball. The force is about 50N. This force is constant. Lets say it took 2 seconds to drop. Then change in time is 2s. How would I integrate that? 50x=mv?

8. Jan 9, 2005

### UrbanXrisis

Sorry, I mean verify

9. Jan 9, 2005

### dextercioby

Aha...That's something else.And it sounds right... :tongue2:

It would be correct,if that "x" would be 2s.I guess it was a typo...

Daniel.