# physics

A steel sphere of radius 6.5 mm is dropped into a lake. What is the terminal speed of the sphere?

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asked by Sarah
1. More information. Terminal speed in the water?

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2. Yes, I think we are supposed to assume that it is in water.

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posted by Sarah
3. The problem here is the assumptions of friction of a small sphere The drag equation is

force drag=densitywater*velocity^2*.47*area

you can calculate area from radius.

at terminal velocity, net force=dragforce

mg-bouyantforce=drag force

now the mass m of the steel ball can be calculated from density steel*volumesphere

bouyant force=densitywater*volumeball*g

do all that, and then you can calculate terminal velocity v of the ball in the water.

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4. Thanks for your help, but I think that it is wrong. I keep getting 1.13 m/s as the answer, so I don't know if I went wrong somewhere.

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posted by Sarah

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