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Old Mar 1st 2010, 07:10 AM   #1
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FREE FALL help

Gonna need help solving this problem.

There are two people both with tennis balls, the 1st person is standing 20meters above the ground, the 2nd person is standing 40meters above the ground. They both throw their tennis balls at the same time with the same initial velocity, the person standing 20meters above the ground throws it up the person 40meters above throws it down, figure out the initial velocity it would take for them to meet at 0meters.
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Old Mar 1st 2010, 08:57 AM   #2
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Hi spanky

1) 40 = Vo t + 1/2 g t^2
2) Consider the first ball to find t in term of Vo. There are 2 cases, when the ball is thrown up and when the ball falls to the ground. Find the tiime taken for the 2 cases and add them to find t in term of Vo.
3) Subs. (2) to (1) and solve for Vo. You'll get nasty equation but I think you still can get Vo
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Old Mar 1st 2010, 10:01 AM   #3
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Originally Posted by songoku View Post
Hi spanky

1) 40 = Vo t + 1/2 g t^2
2) Consider the first ball to find t in term of Vo. There are 2 cases, when the ball is thrown up and when the ball falls to the ground. Find the tiime taken for the 2 cases and add them to find t in term of Vo.
3) Subs. (2) to (1) and solve for Vo. You'll get nasty equation but I think you still can get Vo
ive tried both but i cant seem to get it. also i forgot to mention the answer it is Vo=4 m/s, but i dont know how to get the answer ive tried it all seems quite hard without time nor iniitial velocity.

thanks for ur help though.
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Old Mar 1st 2010, 06:57 PM   #4
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Hi Spanky

I doubt my work will give Vo = 4 m/s , but let's check it.

case 1: 1st ball is thrown upward
the time needed to reach the max. height --> t1 = Vo / g

case 2: 1st ball falls down to ground
*the height reached when it is thrown upward : h = Vo^2 / 2g
*total distance when the ball falls : y = h + 20
*y = 1/2 g (t2)^2 ...(i)

From (i), make t2 as the subject, then t = t1 + t2. Substitute this to equation 1 on the previous post and solve for Vo.
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