# Homework Help: Kinematics Problem.

1. Sep 24, 2004

hi guyes, I was wondering if someone could help me with a problem I had on a quiz. The question hes been puzzeling me for some time now.

A ball is thrown upward in an open roof elevator travelling at 10m/s(elevator speed). The balls speed is 20m/s. How much time does it take for the ball to reach the same point it was thrown at?

2. Sep 24, 2004

### Pyrrhus

You could use galilean transformations to solve this one, the 10m/s of the elevator must be relative to a inertial frame observer, as the 20m/s. Find the speed relative to the elevator, to simplify calculations.

3. Sep 24, 2004

### Staff: Mentor

Is the elevator rising or falling?
I assume that's the ball's speed with respect to the earth/elevator shaft.
I assume you mean same point with respect to the elevator car?

Find the position of the elevator car as a function of time. (Constant speed.)
Find the position of the ball as a function of time. (Constant acceleration.)

Set them equal and solve for the time.