A frictionless ramp of mass 3m is initially at rest on a horizontal frictionless floor. A small box of mass m is placed at the top of the ramp and then released from rest. After the box is released, it slides down the ramp and onto the horizontal floor, where it is measured to have a speed v, having fallen a total distance h.
What is the speed v of the box after it has left the ramp?

Question

A frictionless ramp of mass 3m is initially at rest on a horizontal frictionless floor. A small box of mass m is placed at the top of the ramp and then released from rest. After the box is released, it slides down the ramp and onto the horizontal floor, where it is measured to have a speed v, having fallen a total distance h.
What is the speed v of the box after it has left the ramp?

anonymous · Answer

Well - what have you got so far?

anonymous · Answer

Nothing much. I know that if the block didn't move energy would be conserved. So, 1/2mv^2=mgh. but since it moves I believe energy is lost, but momentum is convserved

anonymous · Answer

1st of all: energy is _always_ conserved. 
2nd: yes, momentum is conserved, but that only gets you so far as: $$p_{block} = - p_{ramp}$$

anonymous · Answer

energy isnt always conserved..it depends on if its elastic or inelastic

i was setting up mv=3mV
mgh=.5mv^2+3/2mV^2

anonymous · Answer

Ahm.. the conservation of energy is probably the most fundamental believe every physicist has. And it holds. ALWAYS!
Whoever told you, it is not conserved in an inelastic collision, lied to you.

Also - there are no colisions in this example.

How did you come up with this? You need to elaborate - not just write down a bunch of stuff..

anonymous · Answer

well i appreciate the help, but what i just wrote down actually got the right answer =D score

anonymous · Answer

Are you trolling?

You tell us, u don't think energy is conserved and then use conservation of energy to solve the example? What the heck?

anonymous · Answer

No, I couldn't remember how to solve it. You were right energy was conserved in this case (sorry i'm still skeptical about always being conserved). I was trying to remember conceptually what i needed to do

anonymous · Answer

well thanks

anonymous · Answer

btw just realized you were talking about energy conservation in general. I was meaning more about just collisions. I know energy isn't can't be lost it can just take different forms.

anonymous · Answer

Well, if it is conserved in general, it is conserved. period. 

And being sceptical is good, but you chose a strange property of the universe to be sceptical about. It is not trivial, I'll give you that and it's more or less some number without any ahm.. deeper meaning (except the conservation of this number), but it has been known quite a while and has been tested countless times. 

anywho.. glad you solved it - keep it up ;)