Question

A friend is riding on the back of a truck that is going away from you at 20 km/h. As the truck passes, your friend throws a ball toward you at 60 km/h relative to her. How fast is the ball going relative to you?

Answer 1

Since the ball moves in the relative direction opposite to the displacement of the car, the speeds must be subtracted, so

`v= 60-20`

`v = 40km/s`

Both move opposite directions. The attached graphic helps to better appreciate the problem in question, which concludes that the relative speed is 40km / s