A running back with a mass of 86 kg and a velocity of 9 m/s (toward the right) collides with, and is held by, a 129-kg defensive tackle going in the opposite direction (toward the left). What is the velocity of the tackle before the collision for their velocity afterward to be zero

6 m/s

Explanation:

The law of conservation of momentum states that the momentum before collision will be equal to the momentum after collision. Momentum is the product of mass and velocity.

Given that the two bodies were moving in opposite direction and the velocity after collision was 0, let the velocity of the tackle before collision be h

Then

(86 * 9) - 129h = (129 + 86) * 0

129h = 86 * 9

h = 86 * 9/129

h = 6 m/s