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

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The discussion revolves around a collision between two pucks on a frictionless air-hockey table, where puck A initially moves at 3.5 m/s and puck B is at rest. After the collision, puck A moves at 2.5 m/s at a 30-degree angle, prompting the need to find puck B's final velocity. The confusion arises regarding the application of energy conservation principles, as one participant questions why energy conservation methods cannot be used in this scenario. It is clarified that since the collision is elastic, kinetic energy is conserved, allowing for the use of conservation of momentum to solve for puck B's final velocity. The conversation emphasizes the distinction between elastic and inelastic collisions in the context of energy conservation.
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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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