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Jun 22, 2008

Prob. 5.178

I have completed this problem, and my alpha term seems huge for the numbers I started with. I was checking to see if any one else had a similar type of answer, or if I should go back and look over my work. It doesn't make sense to me that we would have such a huge acceleration from such a small velocity.
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4 comments:

David Hoff said...

My alpha value was surprisingly large as well but I don't know if it's right or not.

June 22, 2008 at 5:17 PM
ckearney said...

I haven't gone back over my work yet, but my alpha came out in the ten thousands. I personally don't feel like this is reasonable.

However, keep in mind that the numbers will describe the situation so that your intuition doesn't have to. Accelerations can be very brief in systems like this (if you recall the rotating frame videos shown in class):

https://engineering.purdue.edu/ME274/Videos/problem_05_183/

Additionally, the instantaneous acceleration (linear or angular) of a particle does not describe the instantaneous velocity. Think about it in terms of the equations:

Va = Vc + (N)x(Ra/c)
Va = Vo + (va)rel + (w)x(Ra/o)

You can see that acceleration does not appear in the instantaneous velocity equations.

I suggest you check your work anyway. ;-)

June 22, 2008 at 6:39 PM
ckearney said...

Sorry about that link. The link for the rotating reference frames I was talking about can be found on the course web page in the solution for problem 5/183.

June 22, 2008 at 6:44 PM
CMK said...

I agree with ckearney in that it is difficult to assess the accuracy of an acceleration (linear or angular) simply by its magnitude. Very large accelerations are possible for special configurations of mechanisms. These are seen in the animations for 5/183 that ckearney points out.

Also, look at the animations and discussions for 5/145. In this problem you see that links AB and BC have infinite angular accelerations when OA is horizontal.

Having said this, I would recommend that you check back over your work for this problem 5/178. The answer for alpha_ODE for the position shown is 64 rad/sec^2. Not that large.

Let us know if you cannot find your error.

June 22, 2008 at 7:36 PM

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