all objects fall at the same speed?

[esa]

I see, that might be enough proof there...

 

Thanks for the help!!

 

esa

[kyrsjo]

You are quite rigth - a heavier object has a higher terminal velocity.

 

The terminal velocity is when the object is falling at a constant speed - so we can invoke Newtons 1'st (?) law - if the velocity is constant or equals zero, the sum of forces are zero.

 

We have two forces here - the gravitational pull, and the air resistance.

 

If we have an object falling downwards, and we define this direction as the x-axis (vertically downwards), we have:

 

sum F = mg - F_a = 0

 

where F_a is the force due to air resistance - which is pointing upwards (negative x-direction). F_a is not depending on the mass of the object, and a resonable estimate for the size of F_a for a fast-moving object is kv², where k is a constant depending on the size and surface of the falling object. That means our equation now reads:

 

mg - kv² = 0 =>

mg = kv²

 

Note that this equation is only valid when traveling at constant speed/direction. That means that v is the objects terminal velocity.

 

We can revrite the above equation as:

 

v = sqrt(mg/k)

 

where sqrt(...) means the square root of whats inside the paranthesis.

 

So we see that the terminal velocity actually DOES depend on the mass of the falling object.

 

For a more accurate estimate of air resistance, you may read this short paper (norwegian):

http://www.uio.no/studier/emner/matnat/fys...otst_notat2.pdf