# Collisions velocity

1. Aug 30, 2015

### Drizzy

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1. The problem statement, all variables and given/known data

A ball with the mass 0.48kg is hanging on a string. The ball is moved to the side and then released . When the string is vertical, the ball hits the side of a block which is at rest . The ball's position is then 1.5 lower down than from the beginning.

a) the speed gets the ball ?

I used the formulas mgh=mv^2/2 and the answer is 5.4 m/s

b) The block has a mass of 2.5 kg. The block slides 0.65 m on the floor before stopping. The friction coefficient is 0.20. What is the block's initial velocity when hit by the ball?

2. Relevant equations
3. The attempt at a solution

Now this is where it gets tricky. The ball's impulse must be the same as the impulse of the block. The ball's impulse is 0.48 * 5.4 = 2.592 . So that menas that the block's ivelocity 2,592 / 2.5 = 1,0368 m/s But it is wrong and I do not understand why.

One of newtons law says that the impulse is the same in a collision.

Last edited: Aug 30, 2015
2. Aug 30, 2015

### Staff: Mentor

Forget about the ball. You have all the information needed to analyze the block.

You assume the ball stops dead. Don't.

3. Aug 30, 2015

### Drizzy

oh so the impulse is the same only if one of the objet stops?

hmm I have seen some people say that the work of the friction is equal to the ball's energy but I dont understand why

4. Aug 30, 2015

### Staff: Mentor

No, but you do not have enough information to calculate the impulse. At least not yet.

It is friction that makes the block come to rest.

5. Aug 30, 2015

### Drizzy

oh i forgot... thanx so much !

6. Aug 30, 2015

### Drizzy

i think i have enough information because the imoulse is the change in momentum. So the change in momentum of the ball is 5,4^2 * 0,48/2

7. Aug 30, 2015

### Staff: Mentor

Do not assume you know the final speed of the ball after the collision.

That's the kinetic energy of the ball just before the collision.

8. Aug 30, 2015

### Drizzy

okay now i know why thank you