how to calculate air resistance for a falling object? I know without air resistance its 4.9t^2=d but how would you add air resistance? Thanks.

Question

trantom · Answer

oh yea and if it need be you can simplify it by saying the object is a sphere

anonymous · Answer

F = m a = m g - F' 
where F' is the air resistance (usually proportional to density x (velocity)^2.

a) Wait until acceleration stops, then F' = m g = weight of the object.
 
OR

b) plot the trajectory expected from falling, calculate a from fit to y = 0.5 a t^2.  
and get a = 2 y / t^2 = g - F'/m

c) or calculate a = change in velocity per time [from position v. time] = g - F'/m

isaiah.feynman · Answer

I think you would need to consider the drag force(which is air resistance). Its given by the equation$$D=\frac{ 1 }{ 2 }C \rho A V^{2}$$Where C is the drag coefficient 
rho is the air's density
A is cross sectional area of the body falling
and V is the velocity of the falling body.

anonymous · Answer

as far as i remember, drag force also has another component.. which is proportional to V

anonymous · Answer

Yes, this would be useful for prediction.  C is a function of Reynolds number

Re = rho V d / mu = (density) (velocity) (diameter) / viscosity

Can make initial guess and improve iteratively.

I thought "calculate" meant analyze data rather than predict.

anonymous · Answer

If you assume a velocity, then you can calculate a Re number and from that use C as a function of Re to get the resulting drag force. Starting with only the mass and size and shape of the object is harder, as you do not have V and must try to get it to estimate C and use C to estimate V. In some cases, there may be solutions to the  simultaneous problem. I object is not a simple one, sphere preferably, shape and orientation come into play.