# Air Resistance

1. May 5, 2005

### BunHead

let's say i have a 5kg ball and i throw it up with an initial V of 30m/s with air resistance of 25, how would i find the work done by air resistance on the way up and on the way down?

2. May 5, 2005

### whozum

Air resistance of 25? 25 what?

3. May 5, 2005

25N ... Thnx

4. May 6, 2005

### SGT

While the ball is going up you have a total force of (5*9.8 + 25) N acting against the movement. So you can calculate how long it will take for the velocity to be zero and the corresponding height.
When the ball goes down, there will be a force of (5*9.8 - 25) N accelerating it. Knowing the height the ball starts on, you can calculate the time to reach ground.
edited to add:
The work done by air resistance is the force times the diplacement. Or 25N times twice the height.

5. May 6, 2005

### Loozer

Just for fun....how would we calculate the Kinectic energy back to the original place?

6. May 7, 2005

### whozum

Find the height it will go, that distance multiplied by each force acting on it will give the work done with each force. 5*9.8 = 49N from gravity and 25N from air resistance.

Keep in mind though air resistance isnt constant, so saying 25N from air resistance doesnt really make sense.

7. May 7, 2005

### SGT

Once you know the height the ball has attained and the accelerating force down (5*9.8+25)N, you can calculate the final velocity V. The kinectic energy is 5*V2

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