Level 1: Rubber Bullet vs Lead Bullet: Which Pushes the Block Harder?
How much energy vanishes when it sticks?
A 10 g bullet moving at 400 m/s embeds itself in a 1.99 kg wooden block resting on smooth ice. The fraction of the bullet's kinetic energy that is lost (to heat, sound and deformation) is:
Momentum is conserved: 0.01 × 400 = 2.00 × V, so the block and the bullet move off together at 2 m/s. Kinetic energy is not conserved in a collision where the bodies stick together.
Only 0.5% of the energy remains as motion; 99.5% is lost to heat, sound and damage. Momentum conservation fixes the final speed, but it says nothing about keeping the kinetic energy.
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