What was the mass of the cannonball?

Question

Herman is gliding along a handy frictionless train track at a speed of 10.0 m/s. 

Herman and his cart, cannon and cannonball have a combined mass of 450 kg. Herman fires a cannonball straight ahead at 120 m/s, causing the cart to slow down to 8.0 m/s.

Answer 1

This is a momentum conservation question

Assume positive direction to right (and Herman is moving in that direction)

Momentum of system before firing cannonball = Total mass (kg) x speed = 450 x 10 = 4500 kg m/s

This is conserved

If M is mass of cannonball then momentum of system after firing cannonball

= 8 (450-M) + 120M = 4500

From this you can solve for M