Dynamics midterm coming up don't understand pulley questions

AI Thread Summary
The discussion centers on understanding a dynamics problem involving an elevator and pulleys, specifically how to determine distances when motors draw in cable at a constant speed of 5 m/s. Participants clarify that the heights chosen in the textbook are arbitrary points on the cables, which do not affect the overall calculations. The key takeaway is that the focus should be on the rate of change of the cable length rather than the specific distances. The arbitrary points merely serve as reference marks, and the motor speeds dictate the changes in height. Thus, the problem can be approached by concentrating on the motor speeds and their impact on the system.
Faheema Baksh
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Determine the speed of the elevator if each
motor draws in the cable with a constant speed of 5 m>s.

Here is a link to the question and diagram. Scroll down to page 154. Its question 12-199.

http://www.mem.odu.edu/~bawab/ME205f/hwk12.pdf

I don't understand how to get the distances. Any help would be greatly appreciated.
 
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Welcome to PF!

Hi Faheema! Welcome to PF! :smile:
Faheema Baksh said:
I don't understand how to get the distances.

There are motors at B and C, and the cables are disappearing into the motor mechanism …

so we can't measure the distance to the end of the cable! :redface:

So the book has chosen an arbitrary point on each cable, and measured the height of that instead. :biggrin:​
 
Thanks :) but this height that the book has chosen represents what exactly?
 
Nothing!

It's arbitrary! :smile:​

Can't you see from the diagram that they've cut each cable at a totally random point? :wink:

(and if they used some other point, that would only add a constant to the equations, which would cancel out in the end)
 
Yes, it's not the length that matters, it's the rate of change of the length. Imagine that the points where they "cut" the cables are really just black lines marked on the rope. You know the rate of change of the height of the black marks is directly given by the motor speeds, so put the marks wherever you want, and proceed.
 

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