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## Homework Statement

Magnetic puck A, with a mass of 0.100 kg, is pushed towards stationary 0.050 kg

magnetic puck B, to cause a head-on collision. You may neglect friction. The initial

velocity of puck A is 12 m/s [E]. Puck B moves with a velocity of 14 m/s [E], after

the collision.

a) Find the velocity of puck A after the collision.

## Homework Equations

p=mv

pinitial=pfinal

Einitial = Efinal

Ek = 1/2mv^2

## The Attempt at a Solution

Basically I understand how to solve the question using the momentum by why is that I get a different velocity when using the energy?

Since friction is neglected, where could this system be losing kinetic energy?

I did Ei=Ef which is Eki=Ekf

1/2(0.100)(12)^2+0=1/2(0.100)v_A^2 + 1/2(0.050)(14)^2

I end up getting 6.8m/s for the final velocity of puck A

But the correct answer is 5.0m/s

I tried to plug that into the equation but I get

7.2J=6.15J which is incorrect. What am I doing wrong here?