A ball being dropped and bouncing back up?

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A 0.50 kg ball falls from a height of 30m and rebounds to a height of 20m, with the contact time on the ground lasting 2.0 ms. To find the average force exerted on the ball, one must calculate the velocity just before impact and just after rebound using kinematic equations. The initial velocity when hitting the ground is approximately 24.3 m/s, while the rebound velocity is about 19.8 m/s. The change in velocity is then divided by the time to find acceleration, which is used in the formula F=ma to determine the average force. The expected answer for the average force is around 1.1 x 10^4 N upward.
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Homework Statement


After falling from rest at a height of 30m, a 0.50 kg ball rebounds upward, reaching a height of 20m. If the contact between ball and ground lasted 2.0 ms, what a average force was exerted on the ball?

Homework Equations


The kinematic equations?
F=ma
w=mg?
I don't really know

The Attempt at a Solution


Sorry I don't even know where to begin. I don't really understand this section in the book so i don't even know what to do. The answer is supposed to come out to 1.1 x 10^4 upward.
 
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You need to work out the speed the balls hits the floor and then the speed it starts backup at. Acceleration is just change in speed / time.

Hint - the speed it starts back up is the same as the speed a ball dropped from the second height would reach the floor.
 
Ok so the speed of the ball when it hits the ground first time would need the kinematic equation Vf^2= Vi^2 + 2ad right?

Vf^2= (0)^2 + 2(9.8)(30)
Vf^2 = 588
Vf~24.3 m/s ?

Vf^2= 2(9.8)(20)
Vf^2 = 392
Vf ~ 19.8 m/s ?

I think that's correct. correct me if I'm wrong please. How do I find the acceleration?
 
Acceleration = change in velocity / time
Whats the total change in velocity (watch the signs!)
 
Ohhh...

So the acceleration is the change in velocity which would be 19.8 - 24.3/change in time.
Then put it in the F=ma and solve for F correct? or is there more?
 
Correct method - but be careful of the signs.
 
OK thank you very much. I was stuck on this problem for a while.
 
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