# A rock from a boat - could you help me understand?

1. Oct 19, 2012

### Glyper

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

A man on a boat throws behind him a rock of mass m1=2kg with a velocity of v1=18m/s. The mass of a man with a boat is m_2=90kg. What velocty v_2 will the boat go with?

2. Relevant equations

v2 = (m1*v1)/m2

3. The attempt at a solution

So I was upsent on the lesson where we were speaking about this and can't figure out where does the formula I wrote above come from. Why is it certain that m2v2=m1v1? Of course I know I can just plug the numbers inside and bam, I've got my answer but I'd rather know why does it work. Could you help?

2. Oct 19, 2012

### Basic_Physics

Well the eqaution implies that: mom boat - mom rock = 0
Why would that be?

3. Oct 19, 2012

### Staff: Mentor

It comes from the conservation of momentum. Since (presumably), the man and boat start out at rest, the total momentum is zero.

You might want to read this: Momentum Conservation in Explosions

4. Oct 19, 2012

### Glyper

Oh, I see. Thank you a lot!