Why does momentum conservation give m2v2 = m1v1 for a rock thrown from a boat?

[Glyper]

Homework Statement

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?

Homework Equations

v2 = (m1*v1)/m2

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?

 

[Basic_Physics]

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

 

[Doc Al]

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

 

[Glyper]

Oh, I see. Thank you a lot!