# How to calculate time?

1. Sep 29, 2011

### gmxplode

how to calculate time???...

hey guys...just need a little help regarding calculating the time taken by a person if he is allowed to fall from a height of 270 feet tall building...consider weight of the person is 60kg and he is 6 feet tall...now gravity would be constant in real world but how can we calculate the air friction?...

2. Sep 29, 2011

### xts

Re: how to calculate time???...

60kg 6 feet? Must be anorectic...

You can't calculate it precisely - air friction depends on body build, body position, clothes, etc.

You may assume very simplistic model: at first the man falls as there is no air drag (uniformly accelerates), then as his velocity reaches some characteristic value - air drag compensate gravity. It takes 30-60m to reach the final speed.

Final speed is in order of 50-100 m/s, depending on body build and position taken.

See: http://en.wikipedia.org/wiki/Skydiving

3. Sep 29, 2011

### gmxplode

Re: how to calculate time???...

thanks for the reply...but still can we apply any formula over here inorder to find the velocity or the time taken to reach the ground plane?

4. Sep 29, 2011

### BobG

Re: how to calculate time???...

Air drag

$$F_{drag} = \frac{1}{2}\rho A v^2 C_d$$
with ro the air density
A the cross sectional area
v the velocity
C_d the coefficient of drag

A can vary depending on how the falling person is oriented.
C_d is very variable and depends on things such as his orientation (his head is more streamlined than his feet, etc), his clothes, whether he's screaming or if his mouth is closed, etc.

This shows drag coefficients for various objects and for a man falling in a horizontal position (which would be the same as upright in the chart) it lists a drag coefficient from 1.0 to 1.3. I guess the variation depends on whether he's screaming or not (or more likely his build, clothes, etc).

5. Sep 29, 2011

### gmxplode

Re: how to calculate time???...

thanks for the info..but how will i implement gravity with air drag in order to find out the time taken by the person to reach the ground?...can you please elaborate?

6. Sep 29, 2011

### Staff: Mentor

Re: how to calculate time???...

xts' link is good. I Googled skydiver speed versus time (try it yourslef), and got good hits. Here's the 3rd hit on the list: