# More Kinematics

1. Oct 5, 2008

### whyorwhynot

I feel like giving up on learning this, really. I don't understand why I can't understand this, yet it seems so simple.

1. The problem statement, all variables and given/known data
Suppose that you loft a tennis ball over the net with an initial speed of 15.0 m/s, at an angle of 50 degrees above the horizontal. At this instant your opponent is 10.0 m away from the ball. He begins moving away from you 0.30 s later, hoping to reach the ball and hit it back at the moment that it is 2.10 m above its launch point. With what average speed must he move? (Ignore the fact that he can stretch, so that his racket can reach the ball before he does.)

2. Relevant equations

3. The attempt at a solution
Vx = (cos 50)(15) = 9.6 m/s
Vy = (sin 50)(15) = 11.5 m/s

Horizontal:
Vx = 15.0 m/s
x = 10.0 m

Vertical:
a = -9.8 m/s2
Vy = 11.5 m/s

I don't know where to go from there.

Thank you if you can help me.

2. Oct 6, 2008

### Kurdt

Staff Emeritus
First you will need to find how far horizontally the tennis ball moves by the time it reaches 2.1 meters above the launch position. Secondly you will need to compare this to the distance the opponent starts out at and the distance he wants to be at (the distance of the ball you just worked out). Then you can work out how long he has to move that distance and the speed he should move.