# Conservation of momentum

1. Jan 26, 2005

### Gogsey

A block (A) collides with block (B). The blocks were in contact for 0.25 seconds. So what is the force exerted on block (A) by block (B)?

I used the formula for impulse and set it equal to the momentum.
But it requires a change in velocity for it to work. I already know the velocity of each block before and after the collision, so since its the force on (A) by (B), the change in velocity in the formula is the velocity of (B) before and after the collision, right?

2. Jan 26, 2005

### Sirus

Impulse is the change in momentum, not the momentum.

No, when you use that formula, to find the force exerted on block A, you need to use the impulse of block A, so its change in momentum (and velocity, consequently).

3. Jan 26, 2005

### Staff: Mentor

The impulse ($F \Delta T$) equals the change in momentum. To find the force that B exerts on A, you need the change in momentum of A. (Note that the force that B exerts on A is equal and opposite to the force that A exerts on B.)

4. Jan 26, 2005

### Gogsey

The change in momentum of blocK (A) being its mass multiplied by the change in speed(speed after collision minus the speed before collision).

Change in momentum of (A) = impulse will give me the force experienced by block (A) by the force of blaock (B) exerted on it.

5. Jan 26, 2005

### Sirus

Right. Use the expression for impulse given by Doc Al.

Also, momentum is mass multiplied by velocity, not speed. Direction is important.

Last edited: Jan 26, 2005