# Illustration Head-on Elastic Collision Between Light and Heavy Ball at Rest

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Head-on elastic collision of two balls. Before the collision, the first (red) ball has the velocity $$v_1 = 0$$, which means: the ball is at rest. The second (blue) ball has the velocity $$v_2$$ to the left. The mass $$m_2$$ of the second ball is negligible compared to the mass $$m_1$$ of the first ball.

The two balls collide head-on. The first heavy ball hardly moves, i.e. its velocity after the collision is: $$v'_1 \approx 0$$. The second light ball bounces off the heavy ball and flies with the same speed in the opposite direction: $$v'_2 \approx v_2$$.