# Kinetic energy dissipated during inelastic collsion

1. Apr 2, 2014

### jdawg

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

Blocks A and B are moving toward each other. Block A has a mass of 2.0 kg and a velocity of 50 m/s, while B has a mass of 4.0 kg and a velocity of -25 m/s. They suffer a completely inelastic collision. The kinetic energy dissipated during the collision is:

2. Relevant equations

3. The attempt at a solution
So my professor has already worked out this problem, but I'm having trouble understanding why she picked the equations she did and how she manipulated them.

m1v1i-m2v2i=(m1+m2)V

Plug in the numbers and you get V=0

ΔK=$\frac{1}{2}$(m1+m2)V2-($\frac{1}{2}$m1v1i2+$\frac{1}{2}$m2v2i2)

Plug in and get -3750J

2. Apr 2, 2014

### rude man

Almost.
There's no such thing as negative kinetic energy.
The problem did not ask for the CHANGE in k.e. It asked for the k.e. dissipated in the collision.

3. Apr 2, 2014

### jdawg

Wouldn't the negative sign just mean that energy was lost? -3750 J was the answer my professor wrote down.

4. Apr 2, 2014

### rude man

No. Your instructor calculated the CHANGE in k.e. as indicated by the delta K:
" ΔK=1/2(m1+m2)V2-(1/2m1v1i2+12m2v2i2) ""

But the energy DISSIPATED is a positive number The word "dissipated" automatically implies a negative change in energy.

So this is just a semantic issue.

5. Apr 2, 2014

### jdawg

Ohh ok! Thanks, I think it makes sense now :)