physics
posted by Anonymous .
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.

Newton’s 2nd law
vector(ma) = vector(mg ) +vector( N) +vector(Ffr)
Projections:
ma=F(fr)
0=Nmg
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} = … 
nope, 0.733 is wrong

wait, Elena, what is are R and r? Are they the same ones that I used?

R=0.04 m
t= 0.012 m
The solution is correct.May be there is mistake in calculations (or in your answer)... 
net torque on the coin, answer not just force balance