A block and a gun are firmly affixed to opposite ends of a long glider mounted on a frictionless air track. The block and gun are a distance L apart. The system is initially at rest. The gun is fired and the bullet leaves the muzzle with a velocity vb (relative to the fixed air track) and impacts on the block, becoming embedded in it. The mass of the bullet is mb and the mass of the gun-glider-block system is mp.