Momentum Conservation of Turntable and Beetle System

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Homework Help Overview

The problem involves a beetle walking on the edge of a turntable, both initially at rest, and requires determining the angular velocities of both the beetle and the turntable from a stationary observer's perspective. The context is centered around the principles of momentum conservation and rotational motion.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the need for the angular velocity of the turntable to determine the beetle's angular velocity with respect to an observer. There is mention of using a variable for the radius and the relevance of physical equations in solving the problem.

Discussion Status

The discussion is ongoing, with participants exploring the relationships between the angular velocities and questioning the necessity of certain parameters. Some guidance has been offered regarding the use of a variable for the radius, suggesting that it may not be essential for the final solution.

Contextual Notes

There is an implicit assumption that the radius of the turntable is needed for calculations, but participants are encouraged to consider how it may not affect the outcome. The problem is framed within the constraints of a homework assignment, emphasizing the need for a physical understanding rather than purely geometric reasoning.

amb202
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Homework Statement



A beetle with a mass of 30.0 g is initially at rest on the outer edge of a horizontal turntable that is also initially at rest. The turntable, which is free to rotate with no friction about an axis through its center, has a mass of 80.0 g and can be treated as a uniform disk. The beetle then starts to walk around the edge of the turntable, traveling at an angular velocity of 0.0600 rad/s clockwise with respect to the turntable.


a. With respect to you, motionless as you watch the beetle and turntable, what is the angular velocity of the beetle? Use a positive sign if the answer is clockwise, and a negative sign if the answer is counter-clockwise.

b. What is the angular velocity of the turntable (with respect to you)? Use a positive sign if the answer is clockwise, and a negative sign if the answer is counter-clockwise.

c. If a mark is placed on the turntable at the beetle’s starting point, how long does it take the beetle to reach the mark again?


Homework Equations



L= I theta L= mvrsin

The Attempt at a Solution



I know in order to find the angular velocity of the beetle with respect to me, i need to know the angular velocity of the turntable, but without the radius, i cannot figure out how to find this.
 
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Welcome to PF!

Hi amb202! Welcome to PF! :smile:
amb202 said:
A beetle with a mass of 30.0 g is initially at rest on the outer edge of a horizontal turntable that is also initially at rest. The turntable, which is free to rotate with no friction about an axis through its center, has a mass of 80.0 g and can be treated as a uniform disk. The beetle then starts to walk around the edge of the turntable, traveling at an angular velocity of 0.0600 rad/s clockwise with respect to the turntable.
…
I know in order to find the angular velocity of the beetle with respect to me, i need to know the angular velocity of the turntable, but without the radius, i cannot figure out how to find this.

Just call the radius r … you'll find it drops out in the end. :wink:
 
But wouldn't I need the angular velocity of the turntable to figure out the velocity of the beetle with respect to me?
 
Yes, and you can find that from the information given.

This is physics, not geometry …

you can't solve this just with geometry, you need a physical equation also …

which one do you think it is? :smile:
 

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