Why Does the Penny Slide Off the Record at 0.080m?

AI Thread Summary
A penny placed on an accelerating LP record slides off when positioned 0.080m or further from the center due to insufficient friction to provide the necessary centripetal force. The frictional force is calculated using the coefficient of static friction and the normal force, which equals the penny's weight. The centripetal acceleration can be determined from the record's frequency and radius. As the radius increases, the penny's velocity becomes too high, leading to it flying off the record. Ultimately, the mass of the penny cancels out in the calculations, simplifying the problem.
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Homework Statement


A penny is placed on an LP record that is slowly accelerating up to 78 revolutions per minute. It is found that if the penny is placed at 0.080m or greater from the center, then the penny slides off the edge of the record. Find the coefficient of static friction if the mass of the penny is 0.0032kg.


Homework Equations


Ff = mu x n

a = v2/r


The Attempt at a Solution



I don't know how to start. Can someone guide me please??
 
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Ok. Let us begin with this question. You are given the frequency of the LP (and thus the period as well), and you are given the mass of the penny. As you have written above,
Frictional Force = co-eff * Normal and centripetal acceleration = v^2/r = 4Pi^2r*frequency^2. For the penny to stay on the LP, friction has to provide enough centripetal force. When the radius is too great, the velocity of the penny is too large and it flys off the LP. Thus, we are looking for:
Force Friction = Force centripetal = mass of penny* centripetal acceleration.
The rest is plain math and some unit conversions.
 
inutard said:
Ok. Let us begin with this question. You are given the frequency of the LP (and thus the period as well), and you are given the mass of the penny. As you have written above,
Frictional Force = co-eff * Normal and centripetal acceleration = v^2/r = 4Pi^2r*frequency^2. For the penny to stay on the LP, friction has to provide enough centripetal force. When the radius is too great, the velocity of the penny is too large and it flys off the LP. Thus, we are looking for:
Force Friction = Force centripetal = mass of penny* centripetal acceleration.
The rest is plain math and some unit conversions.

thanks.

Normal is just equal to mg, right?
 
yes. So youll notice that the mass does not actually matter in the question since it cancels out.
 
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