Why can't we use cons of energy in this problem? (conceptual)

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SUMMARY

The discussion centers on the collision of two pucks on a frictionless air-hockey table, where puck A (3.0 kg) initially travels at 3.5 m/s and puck B (5.0 kg) is at rest. After the collision, puck A moves at 2.5 m/s at an angle of 30 degrees from its initial direction. The key conclusion is that conservation of energy cannot be applied in this scenario because the collision is elastic, meaning kinetic energy is conserved, but the total energy must account for both momentum and kinetic energy separately.

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This discussion is beneficial for physics students, educators, and anyone interested in understanding the dynamics of collisions, particularly in elastic scenarios.

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Two pucks collide on a frictionless air-hockey table. Initally, puck A is traveling at 3.5 mls and puck B is at rest. After the collision, puck A moves away at a speed of 2.5 m/s and an angle of 30.0 from the initial direction. Find the final velocity of puck B. Puck A has a mass of 3.0 kg and puck B has a mass of 5.0 kg.

I know how to do the problem, but we can't we apply the conservation of energy methods because it is an elastic collision?
 
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In inelastic collisions you can't use conservation of energy.
 
He says that it is an elastic collision.
 

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