A \(4000-\mathrm{kg}\) dump truck is parked on a hill. The parking brake fails, and the truck rolls down the hill. It then coasts briefly along a horizontal stretch of road at \(36 \mathrm{~km} / \mathrm{h}\) before hitting a stationary \(1000-\mathrm{kg}\) car. The car sticks to the grill of the truck, and the two vehicles continue moving forward.

(a) What is the speed of the combined unit immediately after the collision? \((b)\) What is the momentum of the combined unit in the reference frame of a jogger approaching the collision at \(2.0 \mathrm{~m} / \mathrm{s}\) and moving in the direction opposite the direction of the velocity of the moving unit?

(c) How fast, and in which direction, must the jogger be running in order for the momentum of the combined unit to be zero in his reference frame?