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

**1. Homework Statement**

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. Homework 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!!!