# Homework Help: Out of ideas, and frustrated

1. Apr 20, 2008

### aliciagu

Out of ideas, and frustrated!!!!!!!!

1. The problem statement, all variables and given/known data
A ball dropped from a height of 4.00 m makes an elastic collision with the grond. Assume that no mechanical energy is lost due to air resistance. Determine the period of the motion

2. Relevant equations

I know the period (T) = the time required for the object to make one cycle.

3. The attempt at a solution

I get: 4m/9.8 m/s^2=.408 s^2 s=.639 (square root of .408) .639 x 2= 1.278 s (time for it to go down 4 m and up 4 m)
My book gives 1.81 s as the answer. I can get that if I take the square root of 8/9.8 (8 being the total distance traveled in one period) and multiply it by 2. But why? That would be the time required for 2 periods, wouldn't it? I'm banging my head againstthe wall on this one!!!

2. Apr 20, 2008

### EngageEngage

you can calculate the time for it to reach the ground from the kinematic eq:

y = 1/2 at^2. The time for this will be the way down, or 1/2 the cycle.

3. Apr 20, 2008

### aliciagu

That answers it! I knew I was missing something stupid!