# Homework Help: Collision question using conservation of kinetic energy

1. Oct 13, 2008

### tippetmaster

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

A 2.00 kg mas (A) moving at 2.00 m/s strikes another 2.00 kg mass(B) which is at rest. Both are confined to a a straight line to move along.

An impossible outcome for this collision is that A sticks to B and they both move off together at 1.414 m/s. First show that this collision would satify conservation of kinetic energy and then explainwhy this is impossible.

2. Relevant equations
.5(mA)(vAi)2+.5(mb)(vbi)2=.5(mA)(vAf)2+.5(mb)(vbf)2

3. The attempt at a solution
I get that this is a perfectly ineleastic collision but why is it impossible and what does the question mean by "show that this satifies the consevation of kinetic energy" Does the above question even relate to the equation for the consevation of kinetic energy?

ANy help is appreciated thanks!!!

2. Oct 13, 2008

### Rake-MC

It's asking you to show that if they both stick together and move off at 1.414ms-1 then kinetic energy is conserved.

3. Oct 13, 2008

### tippetmaster

okay what does it mean if kinetc energy is conserved?

4. Oct 13, 2008

### Mentallic

Satisfying conservation of kinetic energy means that the total kinetic energy before the collision is more than or equal the total kinetic energy after the collision since no energy can be created and energy could be lost to other forms, such as sounds, heat etc.

What makes you assume this?

5. Oct 13, 2008

### tippetmaster

Because if the colliding objects stick together after the collision then the collision is perfectly inelastic

6. Oct 13, 2008

### tippetmaster

so why is this situation impossible?

7. Oct 13, 2008

### Mentallic

Oh sorry misread the question. Yes this impossible scenario is inelastic.

Simply use the conservation of momentum equation to show this is impossible ~

$$M_aU_a+M_bU_b=M_aV_a+M_bV_b$$ where m=mass, u=intial velocity, v=final velocity.

8. Oct 13, 2008

### Rake-MC

The question is not asking to prove that it's impossible, it's asking to explain why it's impossible.

Is there such thing as a perfectly elastic collision? Why/why not?