Uniform rod pivoting and block/mass energy problem

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  • #1
Alcatrez
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Thread moved from the technical forums, so no Homework Template is shown
A uniform rod of mass m and length l is pivoted at a point on the ground. The rod is initially in vertical position and touching a block of mass M which is at rest on a horizontal surface. The rod is given a push of negligible force and it starts rotating about that point. This causes the block to move forward. The rod loses contact with the block at θ = 30° with horizontal. All surfaces are smooth.
Find M/m

I have conserved the energy, found a relation of

w² = -(alpha)(tantheta)

by considering x=Lcostheta and double differentiating to get a.

I am getting a correct ans of 4/3 but i had considered alpha (ang acc) to be negative which isn't correct, right?

What's wrong with this?
 
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  • #2
haruspex
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had considered alpha (ang acc) to be negative which isn't correct, right?
Assuming you are referring to the angle to the horizontal, that angle will decrease at an increasing rate, so both its first and second derivatives will be negative.
Have I understood your question?
 
  • #3
Alcatrez
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Assuming you are referring to the angle to the horizontal, that angle will decrease at an increasing rate, so both its first and second derivatives will be negative.
Have I understood your question?
Yes. That's exactly what i meant.
 

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