8. A train rolls at a constant speed of 10 m/s across a horizontal track. A child standing on the flatbed of the

train throws a ball so that it appears to move straight up to an observer standing next to the track.

a. What is the horizontal speed of the ball relative to the observer?

b. What is the horizontal speed of the ball relative to the train?

c. If the ball is also thrown upward with a speed of 20 m/s, what is the overall initial speed of the ball

relative to the train?

a) 0 m/s

b) 10 m/s

c) 30 m/s

Explanation:

A). Speed/velocity can be divided into its components (horizontal and vertical), and if an object is moving straight up, that means it only has vertical speed and no horizontal speed, hence the answer is 0 m/s.

B). Since the train is moving at 10 m/s, and there is no horizontal component to the speed of the ball, the ball's speed relative to the train is 10 m/s.

C). This is a bit tricky, because speed has no direction (since it is not a vector) however velocity does. So we have to assume that it has 30 m/s of initial speed. We can also use Pythagoras' theorem and triangles to prove this.