# Homework Help: Inelastic collision?

1. Nov 25, 2009

### sirfinklstin

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

One hockey puck of mass 0.2 kg strikes an identical stationary puck on a frictionless ice rink. If the first puck had a velocity V0 = 3.8 m/s before the collision and V1 = 0.8 m/s after in the same direction, what fraction of the energy was lost?

2. Relevant equations
Conservation of momentum m1v1 + m2v2 = m3v3 + m4v4
Work energy theorem k = (1/2) mv^2 or (mv)^2/2m

3. The attempt at a solution
3.8 + 0 = .8 + 3
(I left out mass becuase all objects have the same mass.)

I am pretty much stuck becuase I do not know the final velocity of V2, and if there was energy lost then V2 shouldn't equal 3 m/s.

2. Nov 25, 2009

### tiny-tim

Welcome to PF!

Hi sirfinklstin! Welcome to PF!
Why not??

3. Nov 25, 2009

### vishal007win

u almost solved the problem...
v1= 3.8
v2=0...bcoz stationary given
v3=0.8
and v4 u have to find out...final velocity of other mass...
which u got 3

now jus find difff between initial and final energy
wats d problm??

4. Nov 27, 2009

### sirfinklstin

Ok, thanks for the help with that, i thought a loss of energy would mean less than 3 m/s for V4, here is what I have now -

puck 1 before collision
(1/2)(.2)(3.8)^2
1.444

puck 2 before collision
(1/2)(.2)(3.8)^2
0

puck 1 after collision
(1/2)(.2)(.8)^2
.064

puck 2 after collision
(1/2)(.2)(3)^2
.9

loss of energy = .48J

is this correct?

5. Nov 27, 2009

### frustr8photon

looks good! just convert that into a fraction...

6. Nov 27, 2009

### sirfinklstin

(48/100)j ?

7. Nov 27, 2009

### frustr8photon

"what fraction of the energy was lost?"

8. Mar 23, 2010

### huybinhs

So, how to calculate fraction of the energy was lost? I'm lost :(