A 30 kg child moving at 4 m/s jumps onto a 50 kg sled that is initiall

[chicagobears]

Homework Statement

A 30 kg child moving at 4 m/s jumps onto a 50 kg sled that is initially on a frictionless surface. Find the speed of the child-sled system after it jumps onto the sled
I know the solution looks like this
(30)(4)=(30+50)(v)
but i don't know why and what that equation is?

 

[PhanthomJay]

You should look up momentum or collisions thru a web search or text. It is the conservation of momentum equation for a 'totally inelastic' collision. Please indicate why this collision is totally inelastic; but in general, momentum is always conserved in any type collision when no external forces are present.

 

[chicagobears]

Thanks PhantomJay. I read about it and it makes sense. We haven't learned momentum yet so I was confused on what they were doing.