A flying bullet pierces the board. How does the kinetic energy of a bullet change? Doesn’t this change in energy

A flying bullet pierces the board. How does the kinetic energy of a bullet change? Doesn’t this change in energy contradict the law of conservation of energy?

Punching the board, the bullet does work against frictional forces by changing its kinetic energy. The bullet speed decreases. There is no contradiction with the law of conservation of energy, since the bullet and the board heat up in this case.

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