Impulse Momentum Method for Rotational

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Homework Help Overview

The discussion revolves around the impulse momentum method in the context of rotational dynamics, specifically regarding the interaction between two shafts, A and B, and how their moment of inertia can be combined. Participants explore the implications of treating the engagement of the shafts as a collision and the conditions under which impulse is considered zero.

Discussion Character

  • Conceptual clarification, Assumption checking, Exploratory

Approaches and Questions Raised

  • Participants question the equivalence of the engagement of shafts to a collision, seeking clarification on the conditions that allow for impulse to be zero. There is also discussion about the rationale behind combining the moment of inertia of the two shafts to analyze the problem.

Discussion Status

The conversation is ongoing, with some participants providing insights into the relationship between angular momentum and external torque. Guidance has been offered regarding the determination of positive and negative momentum directions, but no consensus has been reached on the initial questions posed.

Contextual Notes

Participants are navigating assumptions about the nature of the interaction between the shafts and the definitions of momentum in this context. There is an emphasis on understanding the absence of external torque and its implications for the system's angular momentum.

freshbox
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Homework Statement


For impulse momentum, I know that collision between two object, impulse = 0.
Can somebody explain to me how come for this question the impulse is 0 "shaft A engaged to shaft b" has the same meaning as collision?

And I assume that it is right for me to add shaft A and shaft b moment of inertia together in order to solve the question but can somebody explain the reason behind it?Thanks a lot.
 

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freshbox said:

Homework Statement


For impulse momentum, I know that collision between two object, impulse = 0.
Can somebody explain to me how come for this question the impulse is 0 "shaft A engaged to shaft b" has the same meaning as collision?

And I assume that it is right for me to add shaft A and shaft b moment of inertia together in order to solve the question but can somebody explain the reason behind it?


Thanks a lot.

impulse momentum = angular momentum. For a rigid body, angular momentum = (moment of inertia) multiplied by the (angular velocity ω).
In a collision of point masses, the momentum (mv) stays constant. In case of interaction between rigid bodies, the moment of inertia stays constant if there is no external torque.
The two shafts rotates around the same axis, but slipping one on the other. That is some interaction between them, like in a collision. The interaction will speed up the rotation of the faster shaft and slow down the rotation of the other one, till they both rotate with the same angular velocity.
The angular momentum of a system can change if some external torque acts on it. There is no external torque in this case, so the sum of the angular momenta of the shafts stays constant.

ehild
 
Can you give some example of external torque?
 
You need an external force F which is not parallel to the axis of rotation and its line does not go through the axis. You get the torque τ as force times the length of its arm d, τ=Fd. The "arm" is the distance of the line of force from the axis. When you push a door open, you exert a torque on it. The door interacts with the hinge, but your force causes an external torque.

ehild
 

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Thank you ehild. For the next question, how do you determine which momentum is -ve or +ve? Bigger momentum = +ve?
 

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In this problem you decide which direction you take positive. You can choose the greater one.

ehild
 
ok thank you
 

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