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r/HomeworkHelp•
junriee

[University Physics I] Angular Momentum/Rotational Kinematics

Mass m_1 moves with speed v in a circular path of radius R on a frictionless horizontal table. It is attached to a string that passes through a frictionless hole in the centre of the table. A second mass m_2 is attached to the other end of the string. Derive an expression for R in terms on m_1, m_2, v and g.

Here's a diagram of what this would look like: http://i.imgur.com/zbni3pk.jpg

I'm pretty sure I should approach this from an angularmomentum perspective but I'm not sure how to find an expression for the angular momentum of m_2. Any help would be appreciated. Thank you!!

Posted Apr 21, 2016, 9:26 AM UTC

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r_a_bot•

This is not related to the angular momentum of m_2, it is all about the gravity of m_2 being cancelled out by the centrifugal force of m_1, this video shows an example with a tennis ball and a brick

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nmacholl•

The weight of the hanging m2 is what provides the centripetal acceleration for the rotating m1.

Since the centripedal acceleration (ac ) is given by:
ac = v^2 /R

By Newton's laws we know that (a) a = F/m, and (b) the weight of m2 is the centripedal force experienced by m1.

ac = F / m1
F = m2 g

It is also worth mentioning that there is no centrifugal force. Typically that is an artifact of not working in an inertial reference frame in which case Newton's laws don't apply anyway.

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