Calculate Time with Weight and Height | Air Friction Formula

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Discussion Overview

The discussion revolves around calculating the time taken for a person to fall from a height of 270 feet, considering factors such as weight, height, and air friction. Participants explore the complexities of incorporating air drag into the calculations alongside gravity, touching on both theoretical and practical aspects of the problem.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about calculating the time of fall from a height of 270 feet, given a weight of 60 kg and a height of 6 feet, while questioning how to account for air friction.
  • Another participant suggests that precise calculations are difficult due to the variability of air friction, which depends on factors like body build, position, and clothing.
  • A simplistic model is proposed where the person initially falls without air drag and then reaches a terminal velocity where air drag balances gravity, taking 30-60 meters to reach this speed.
  • Discussion includes a formula for air drag, highlighting that the drag force is dependent on air density, cross-sectional area, velocity, and the drag coefficient, which varies based on orientation and clothing.
  • Participants express uncertainty about how to combine gravity with air drag to determine the time taken to reach the ground, seeking further elaboration on the implementation of these factors in calculations.

Areas of Agreement / Disagreement

Participants generally agree on the challenges of calculating the fall time due to the complexities introduced by air drag, but no consensus is reached on a specific method or formula to combine gravity and air drag in the calculations.

Contextual Notes

Limitations include the dependence on various assumptions regarding body orientation, clothing, and the variability of drag coefficients, which complicate the calculations.

gmxplode
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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?...
 
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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
 


xts said:
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

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?
 


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).
 


BobG said:
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).
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?
 


gmxplode said:
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?...

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:

http://www.google.com/url?url=http:...s+time&usg=AFQjCNE7UkhtKsEl-H2m9nlugYEUBgs_pQ

.
 

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