Answer to Question #191147 in Classical Mechanics for Mike

Question

75 kg ball player moving at a velocity of 7.8m/s hits a player moving in the opposite direction who has a mass of 90 kg. If they collide and stop to have a speed of zero how fast is the other player moving originally?

Answer 1

We can find the initial velocity of the other player from the law of conservation of momentum:

$$p_i=p_f,$$ $$m_1v_{1i}+m_2v_{2i}=0,$$ $$v_{2i}=-\dfrac{m_1v_{1i}}{m_2}=-\dfrac{75\ kg\cdot7.8\ \dfrac{m}{s}}{90\ kg}=-6.5\ \dfrac{m}{s}.$$

The sign minus means that the other player moves in the opposite direction before the collision.

Answer:

$v_{2i}=6.5\ \dfrac{m}{s}$, in the opposite direction.