Work done by Air resistance on ball?

1. Feb 6, 2015

Ivar

1. The problem statement, all variables and given/known data

Hey! Problem:

A ball falls 8,5 meters, and hits the ground with 11 m/s.
The ball has a mass of 0,2 kilos.

How much work was done by Air resistance on the ball?

2. Relevant equations
I guess mechanical energy eqution?

3. The attempt at a solution
answer should be -4,5 joule? I think. I came to 70 or something. Someone help?

2. Feb 6, 2015

3. Feb 6, 2015

Ivar

I did this:Wa=1/2*0.2*11^2 + 0.2*9.81*8.5 - 1/2*0.2*0^2 = 12.1+16.67=28,77

How do I Get there?

4. Feb 6, 2015

Nathanael

What about when the work from air is zero? What speed would the ball hit the ground with? Does your equation correctly predict this?

5. Feb 6, 2015

Ivar

Well, that would be the 1/2v^2*mgh=1/2V(0)^2*mgh(0), would it not? How does that help me calculate the resistance of the air?

6. Feb 6, 2015

Ivar

Must i find the speed with no Air resistance, calculate with the same equation and subtract the first answer from my second one? Which, to hope, will give me an answer of -4,6 Joule?

7. Feb 6, 2015

Nathanael

No that's not right. The right hand side of that equation is always zero. But surely the final speed is not zero? You seem to mixing up the kinetic energy with the potential energy, I would suggest reviewing these concepts.