Posted by **Anonymous** on Sunday, July 28, 2013 at 9:36pm.

A U.S quarter is rolling on the floor without slipping in such a way that it describes a circular path of radius R=4 cm. The plane of the coin is tilted at an angle of θ=45∘ with respect to the horizontal plane (see the figure below). Find the coin's period T in seconds, that is, the time it takes for the coin to go around the circle of radius R. The radius of a U.S quarter is r=1.2 cm.

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**Elena**, Monday, July 29, 2013 at 5:49am
Newton’s 2nd law

vector(ma) = vector(mg ) +vector( N) +vector(Ffr)

Projections:

ma=F(fr)

0=N-mg

mv²/r =μN

N=mg

mv²/r = kN = μmg

μ=v²/gr.

tanα =F(fr)/N = μmg/mg = μ = v²/gr.

v=sqrt{gr•tanα}

T=2πR/v= 2πR/ sqrt{gr•tanα} = …

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**Anonymous**, Monday, July 29, 2013 at 8:03am
nope, 0.733 is wrong

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**Anonymous**, Monday, July 29, 2013 at 8:13am
wait, Elena, what is are R and r? Are they the same ones that I used?

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**Elena**, Monday, July 29, 2013 at 5:58pm
R=0.04 m

t= 0.012 m

The solution is correct.May be there is mistake in calculations (or in your answer)...

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**s**, Monday, July 29, 2013 at 6:36pm
net torque on the coin, answer not just force balance

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