Dynamics - Very simple Q; Just need clarification

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SUMMARY

The discussion centers on the dynamics of a stationary point D on a disc, specifically addressing the equation vD = vB - vD/B. The key conclusion is that since point D is stationary ('without slipping'), its velocity vD is set to zero. This leads to the equation 0 = vB - vD/B, which is crucial for solving related dynamics problems. The explanation clarifies why this assumption is consistently applied across various solutions.

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Homework Statement


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Homework Equations


Skipping this, as I just have a simple question

The Attempt at a Solution


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Here is the full solution, but I just have one simple question about a simple step. On the part for disk B, they establish the equation vD = vB - vD/B. In the proceeding step, they set vD = 0. I have checked out a few other solutions online and they ALL do this, but none explain. I'm curious as to why this happens. Thanks!
 
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D is the point of contact between the large disc and the outside surface. It is specified to be stationary ('without slipping'), so its velocity vD is zero. That velocity is also equal to vB-vD/B, as the first term is the linear velocity of B, the centre of the big disc, and the second term is the linear velocity of D relative to B.

Putting those two facts together gives an equation 0 = vB - vD/B, which can then be used to work out the rest.
 
Thank you andrewkirk, that makes sense.
 

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