SOLUTIONS TO EXERCISE 19

The momentum of the comet is p = mv = 500 * 25 = 12500 ton km/s.
The momentum of the planet is p = 5 billion * 0 = 0 ton km/s.
The total momentum of the comet-plus-planet system is thus 12500 ton km/s.

When the comet hits and sticks to the planet, the planet now has mass
m = (5,000,000,500 tons).  Because of Newton's First Law, the total
momentum stays the same; so the planet must have p = 12500 ton km/s.

Thus, since p = mv,  v = p/m = (12500 ton km/s) /  (5,000,000,500 ton)
= 0.0000025 km/s = 0.0025 m/s.  (This speed is, by the way, about 8 inches per 
day.) The direction of the motion is the direction the comet was travelling before 
the collision.