# Homework Help: Balls dropped and thrown at the same time

1. Mar 3, 2012

### wmsaqqa

A ball is thrown straight up from the edge of the roof of a building. A second ball is dropped from the roof a time of 1.14 later. You may ignore air resistance.

A- If the height of the building is 19.3 , what must the initial speed be of the first ball if both are to hit the ground at the same time?

B- Consider the same situation, but now let the initial speed of the first ball be given and treat the height of the building as an unknown. What must the height of the building be for both balls to reach the ground at the same time for v0 = 8.80 .

i need a help for this question please.. 3 hours and im getting a wrong answers..

i used these formulas : y=v0.t-1/2.g.t2 , and this : y=1/2.(v0-v).t

Last edited: Mar 3, 2012
2. Mar 3, 2012

### HallsofIvy

Re: Very difficult question for me.. and i need a help

It would help if you showed what you have done. Yes, y= v_0t- (1/2)gt^2 will help here. The second ball is dropped- i.e. has v_0= 0. So when will that ball hit the ground? Set t equal to that for the first ball and solve for v_0.

3. Mar 3, 2012

### wmsaqqa

Re: Very difficult question for me.. and i need a help

for A .. i got this answer : -22.5
right?

Last edited: Mar 3, 2012
4. Mar 3, 2012

### wmsaqqa

Re: Very difficult question for me.. and i need a help

for B.. i got the answer -3.66

and both the answers are wrong.. !!

5. Mar 3, 2012

### wmsaqqa

Re: Very difficult question for me.. and i need a help

I need a help and check for my answers.. and i showed you the formula and you said that this formula is right.. but the answers are wrong..