Physics Free Fall questions - please check over my work.?

Question

I am doing a physics lab and included are these two questions but I'm not positive I've done my calculations correctly.

First question is: If a 2.50kg physics book is dropped from rest at a height of 1.50m, how much time would it take the book to hit the ground? Assume no air resistance.

Second question is: If a 2.50kg physics book is dropped with an initial velocity of 10.0m/s from a height of 1.50m, how much time would it take the book to hit the ground. Assume no air resistance.

My calculated answer for the first questions was .55 seconds and for the second question .16 seconds.

If anyone can work these and let me know if I've done this correctly I would appreciate it very much. If my answers are in fact incorrect if you can put down your calculations that would be a big help so I can see what I am doing wrong.

Thanks to anyone who can help.

Answer 1

Assuming you use 10m/s-1 for g:
1. t = sqr ( 2 * e / g ) where e = 1.5m
Gives t = sqr ( 2 * 1.5 / 10 ) = 0.5477 s
2. without trying to be clever, imagine it falls from higher, starting with no init velocity:
- time to reach 10m/s: t = Vi / g = 10 / 10 = 1s
- fall distance to reach 10m/s: e = g * t² / 2 = 10 * 1² / 2 = 5m
- total distance of the fall: 5m + 1.5m = 6.5m
- total time to fall from 6.5m: T = sqr ( 2 * e / g )
= sqr ( 2 * 6.5 / 10 ) = 1.1402 s
Time to fall the last 1.5m = 1.1402 - 1 = 0.1402s
You can review with g = 9.81.

Answer 2

I'd help, but I failed physics. Take my advice, don't partner up with the hottest girl in the class...