Homework Help: Totally elastic and momentum collision equation help! Thanks

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

A 12kg wad of clay is hurled horizontally at a 100g wooden block initially at rest on a horizontal surface. The clay sticks to the block. After impact, the block slides 7.50m before coming to rest. If the coefficient of friction between the bloack and the surface is .650, what is the SPEED of the clay immediately before impact?

2. Relevant equations

This is a perfectly inelastic collision correct?

My book tells me in the answer section to use the following formula, but Im not sure where it came from or why to use this. Am I to use this for equations related to inelastic collisions and friction involved?

1/2(m1+m2)V2^2 = friction(m1+m2)gL

How did it come up with that???? I guess I am just having trouble figuring out how I was suppose to come up with that formula.

Any clarification?



3. The attempt at a solution

 

Ok great.

When looking at this problem... What is the first and second things I should be trying to figure out. Mainly, when looking at this problem specifically, what would I be looking to do first?

I want to systematically break this equation down from start!

 

I really am not sure... ?

 

I thought for step one, I would use:

Vf = m1V1i + m2V2i / (m1 + m2)

?

 

Vf would be velocity of the system?

v1i would be for the clay and v2i would be wood ???

 

Ok, so first thing I do when looking at this is figure out the Vf of the system after the collision.

With the formula I supplied for the first step Vf = m1V1i + m2V2i / (m1 + m2) , how do I go from there, to here: 1/2(m1+m2)V2^2 = friction(m1+m2)gL

 

Doc? Can you explain the above reply to me if you can, or anyone else? :)

 

Oh ok Doc, im starting to understand it a little better. Just having trouble coming to a conclusion on what exact formula to use. Like what stpes I take to get the right formula....

You ask what is V2 ? Well, thats the Velocity of the wood, which is zero, correct? What significance is that may I ask?

Again, MUCH thanks for the help!

EDIT: I guess what im missing is how I would come to the formula to get the speed of the system, as detailed here: 1/2(m1+m2)V2^2 = friction(m1+m2)gL

I have never seen that before!