Question

i need help solving this problem. A projectile of 1.0kg mass approaches a stationary body of 5.0kg at 10.0m/s and, after colliding, rebounds in the reverse direction along the same line with a speed of 5.0m/s. what is the speed of the 5.0kg body after the collision?

Answer 1

Well, this is clearly a momentum problem. First note, the collision is elastic...that is, the objects are bouncing off of eachother. Second, note that this is a one-dimensional problem and that momentum will be conserved before and after the collision.

First, figure out the initial momentums of each object:
P = mass * velocity

object 1: 1kg * 10 m/s = 10 kg*m/s.
object 2: 5kg * 0m/s = 0 kg*m/s

Therefore, the total initial momentum is 10 kg*m/s + 0 kg*m/s = 10 kg*m/s

Now see what you have after the collision
object 1 = 1kg * -5 m/s = -5 kg*m/s
object 2 = 5kg * V

Since Pf must = Pi:
10kg*m/s = -5 kg*m/s + 5kg*V

V is therefore 3 m/s for the 5kg object