Question

A blue and a green billiard ball, each with a mass of 0.15 kg, collide head on. Before the collision, the blue ball had a speed of 3 m/s while the green ball had a speed of 2 m/s in the opposite direction. After the collision, the blue ball stays in place while the green ball continues in motion.

In 3-4 sentences, represent the situation before and after the collision and calculate the speed of the green ball after the collision. Be sure to discuss direction.

Answer 1

Let +x be the original direction of the blue ball. Initially: m = 0.15 kg, v_blue,i = +3.0 m/s, v_green,i = −2.0 m/s. After the collision: v_blue,f = 0 and v_green,f = ?; by conservation of momentum m(3.0) + m(−2.0) = m(0) + m v_green,f so v_green,f = 3.0 − 2.0 = +1.0 m/s. Thus the green ball moves at 1.0 m/s in the same direction the blue ball was originally moving (i.e., opposite to the green ball’s initial direction).