A penny is sitting on the edge of an old phonograph disk that is spinning at 30.

Question

A penny is sitting on the edge of an old phonograph disk that is spinning at 30.0 rpm and has a diameter of 20 cm. What is the minimum coefficient of static friction between the penny and the surface of the disk to ensure that the penny doesn't fly off? When a mass m sits at rest on a spring, the spring is compressed by a distance d from its undeformed length (Fig 1a). Suppose instead that the mass is released from rest when it barely touches the undeformed spring (Fig 1b). Find the distance D that the spring is compressed before it is able to stop the mass. Is d = D?. If not, why not?

Explanation / Answer

1)w=3.14 rad/s

now balancing the forces,

w^2r=ug

or 3.14^2*0.1=u*9.8

or u=0.1

2)when it is at rest,

d=mg/k

when it is released from u ndeformed length,

D=2mg/k

hence D=2d

it is because we need to apply the conservation of energy in the d case to get the result since when D=d, the block will have some kinetic energy associated with it.

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