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basic physics - ball bearing falling through oil

andieje
andieje used Ask the Experts™
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Hi

If you had a ball bearing falling through a cyclinder of oil until it reached terminal what is the name of the force opposing the motion of the ball bearing? Is it friction? Presumably whatever this force is increases with the speed of the ball bearing in the same way that the amount of air resistance increases with the speed of a falling object?

If you had a ball bearing dropping through a tall cyclinder of oil how would you show it had reached terminal velocity? Is that just showing when it starts travelling at a constant speed?

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Viscosity, which is proportional to velocity to the 4th power.
Air resistance is also viscosity.
Yes.  It is friction (oil, air, water all have different viscosities -- or thicknesses -- but friction is the force that a falling object feels when falling through anything other than space).  And yes, you can detect terminal velocity simply by seeing when the object stops accelerating or in other words, reaches a constant speed.  At that point the forces are equal.
Think of viscosity like sand paper.  When something rubs against it, it will feel friction...the rougher the sandpaper the greater the friction.  Oil is much thinker, and therefore resists more.

see: http://en.wikipedia.org/wiki/Viscosity
and
http://www.daviddarling.info/encyclopedia/V/viscosity.html
ozo
Most Valuable Expert 2014
Top Expert 2015

Commented:
viscous drag is linear with velocity
at high Reylolds number, drag is quadratic with velicity
" Is that just showing when it starts travelling at a constant speed?"
That is a definition of terminal velocity
"viscous drag"    ----    an excellent way of putting the phenomenon .
ozo
Most Valuable Expert 2014
Top Expert 2015

Commented:
viscous drag is not proportional to velocity to the 4th power.