# Conceptual understanding of circular motion

#### cmwilli

[PLAIN]http://img207.imageshack.us/img207/37/capturebg.jpg [Broken]

a=v^2/r
v=2pir/T
f=ma

So I already have the answers to all the questions. The problem I am having is I don't understand why the centripetal force is not equal to the weight of the person. If it isn't the weight what force, other than gravity, is acting on him that would be called the centripetal force.

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#### tiny-tim

Homework Helper
hi cmwilli!
… I don't understand why the centripetal force is not equal to the weight of the person. If it isn't the weight what force, other than gravity, is acting on him that would be called the centripetal force.
i think the book is correct, but rather confusing

the F = ma equation for the radial (vertical) direction, for someone standing on the ground, is mg - N = mv2/r

the total centripetal force is the LHS, mg - N (so, for no rotation, it would be zero), but the weight (ie, what would show up if he was standing on a weighing machine) is the reaction force N, which is less than mg

the book is confusing because we usually call mg the weight, although technically our weight is what we feel, ie the reaction force, N, against us (which is usually the same as mg, but not in this case!)

#### cmwilli

What does LHS mean?

#### tiny-tim

Homework Helper
Left-hand side!

(of the equation)

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