Physics

A 2kg mass moving with a velocity of 7 m/s collides elastically with a 4 kg mass moving in the opposite direction at 4 m/s. The 2 kg mass reverses direction after the collision and has a new velocity of 3 m/s. What is the new velocity of the 4 kg mass?

My first approach was with conservation of kinetic energy.
Then I got the correct answer using the conservation of momentum formula.

However, why do I not get this same answer using conservation of kinetic energy? Both kinetic energy and momentum are conserved in an elastic collision...I'm accounting for direction changes in the conservation of kinetic energy formula as well...

And since apparently I cannot use conservation of kinetic energy for this, when DO I use conservation of kinetic energy?

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1. The problem is fake. It is not an elastic collision. You can prove that by not assigning a velociy for after the collision, then solving for it using both equations.

conservation of momentum holds for elastic AND inelastic equations.

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bobpursley
2. Anyway, ALWAYS use momentum first. You can not count on the energy being the same before and after unless the collision is elastic. I suspect that the claim of elasticity was a typo.

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