500
If a 5 kg ball traveling at 5 m/s hits a stationary 2 kg ball and the collision is perfectly elastic, this is how fast the 2 kg ball will travel.
What is 12.5 m/s? (The momentum of the 5 kg ball equals 5 kg x 5 m/s = 25 kg m/s. This momentum is fully transferred to the 2 kg ball. So 2 kg x ?? m/s = 25 kg m/s 25 kg m/s divided by 2 kg = 12.5 m/s)