The full drum is rolling forward with a velocity of 2m/s and collides with the stationary empty drum. Then after the collision both drums move forward together. Calculate the velocity of the drums after the collision.

Momentum = mass x velocity

Before collision:

Momentum of full drum = 100kg x 2m/s = 200kg m/s

Momentum of empty drum = 20kg x 0m/s = 0kgm/s

Total momentum before collision = 200 kg m/s + 0 kg m/s= 200 kg m/s

Principle of conservation of momentum states that the momentum before a collision is equal to the momentum after the collision

Total momentum after the collision = 200 kg m/s

Now they are moving together, combine the masses 100kg + 20kg = 120kg

Velocity = momentum/mass

Velocity after collision = 200/120 = 1.67 m/s or 1.7 m/s

This seems really long and complicated, but its not that bad.