# Work done by Air resistance on ball?

## Homework Statement

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?

## Homework Equations

I guess mechanical energy eqution?

## The Attempt at a Solution

answer should be -4,5 joule? I think. I came to 70 or something. Someone help?

Nathanael
Homework Helper

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?

Nathanael
Homework Helper
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

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

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?

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?

Nathanael
Homework Helper
Well, that would be the 1/2v^2*mgh=1/2V(0)^2*mgh(0), would it not?
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.

How does that help me calculate the resistance of the air?