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Suppose we have an object and we throw it straight upward in the air. How do we apply the Air friction force to this object while moving upwards and after that downwards? Sorry if it's easy because I'm still a novice!

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Hi Goodarz - what are you confused about? Is there some reason you think air friction could not apply to this object? –  David Z Oct 4 '12 at 16:33
    
I simply didn't know how to apply Air friction force to the objects! All the problems I've solved before supposed that the Air friction could be omited... –  Goodarz Mehr Oct 4 '12 at 21:00
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You can make a good estimate of the force of air resistance from

enter image description here

Where p is the density of air, v is the speed, A is the cross section area and Cd is a factor depending on the shape (ie how streamlined) of the object - you can look this up for simple shapes.

The tricky bit of the equation is that the drag depends on speed, which is constantly changing, and then the drag also changes, which changes the speed.... The easiest way to deal with this is to do it numerically, work out the speed at each point in a spreadsheet/computer program, then work out the drag at that point and adjust the force.

Remember also that the drag always acts to slow the object, so on the way up it acts doen, and on the way down the force acts up!

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