# Homework Help: Collision of pucks on frictionless table

1. Nov 28, 2004

### disruptors

On a frictionless horizontal air table, puck A (with mass 0.366 kg) is moving toward puck B (with mass 0.254 kg ), which is initially at rest. After the collision, puck A has velocity 0.119 m/s to the left, and puck B has velocity 0.655 m/s to the right.

a) What was the speed of puck A (Vai) before the collision?
I got 0.336 m/s which is correct by using m1*v1 + m2*v2=m1*v -->elastic collision

B)Calculate , the change in the total kinetic energy of the system that occurs during the collision.

for the energy part i am using the equation

1/2 m1 v1(squared)i = 1/2 m1 v(squared)1f + 1/2 m2 v(squared)2f

question is looking for change in kinetic energy DURING collision

i calculated Final KE - Initial KE but is that not considered the total change in KE DURING the collision? as i got 0.057 J.

2. Nov 28, 2004

### Staff: Mentor

OK, but that's conservation of momentum. (No assumption of elastic collision needed.)

That equation assumes that energy is conserved. Don't assume that. Instead, calculate the final KE (right side of your equation) and the initial KE (left side). Then find the difference (final - initial); that's the change in KE.

3. Nov 28, 2004

### disruptors

thanks, i did that again and got it correct after i saw i made a calculating error