Plz help PHYSICS STUCK CIRCULAR MOTION?

Question

1) A coin is placed 14.5 cm from the axis of a rotating turntable of variable speed. When the speed of the turntable is slowly increased, the coin remains fixed on the turntable until a rate of 40 rpm is reached, at which point the coin slides off. What is the coefficient of static friction between the coin and the turntable?

2) Suppose that a binary star system consists of two stars of equal mass. They are observed to be separated by 340 million kilometers and take 5.0 Earth years to orbit about a point midway between them. What is the mass of each?
______ kg

3) During an Apollo lunar landing mission, the command module continued to orbit the Moon at an altitude of about 90 km. How long did it take to go around the Moon once?
____ min

civil_av8r · Accepted Answer

1) Sum of the forces in the vertical direction = 0 = N - m*g
N = m*g

Sum of the forces in the horizontal direction = 0 = Centripetal Force - friction

Centripetal Force = m*v^2/r
Friction = u*N = u*m*g

Centripetal force = friction
mv^2/r = u*m*g

u = v^2 / (r*g)

To solve for v, use the rate.  You need to convert 40 rpms into rad/s

w = 40 rpms = 40 rev / min * ( 2*pi rad / 1 rev ) * ( 1 min / 60 sec )

Then v = w*r, Sub into 'u'

u = w^2*r / g

2) F_gravity = Centripetal force = G*M^2 / R^2 = M*V^2 / R

The distance you are given is the diameter, divide by 2 to get R

**EDIT: Actually for Gravity, R = diameter vs R for centripetal, R = radius**

M = V^2*R / G

3) Seems like we need some more info.

? · Answer

b. A pail of water on the top of a rope swung in a around action with the stress on the rope offering thecentripetal acceleration of the backside of the pail against the water to maintain it from falling on the guy swinging the pail of water. See the 2009 Yahoo solutions in the source.

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Plz help PHYSICS STUCK CIRCULAR MOTION?

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