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GATE 2017 ME (ME1) – Question 51

Engineering Mechanics · Impulse and momentum, energy formulations · 2 marks · Multiple choice

Two disks A and B of equal mass m and radius R roll without slipping from rest down identical inclined planes. A has all mass concentrated at its rim, while B is uniform. At the bottom, the ratio of centre speeds of A to B is

  1. $\sqrt{3/4}$
  2. $\sqrt{3/2}$
  3. 1
  4. $\sqrt2$

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Correct answer: (A) $\sqrt{3/4}$

Explanation

Energy conservation gives $mgh=mv^2/2+I(v/R)^2/2$, hence $v^2=2gh/[1+I/(mR^2)]$. A has $I_A=mR^2$ and B has $I_B=mR^2/2$. Thus $v_A/v_B=\sqrt{(1+1/2)/(1+1)}=\sqrt{3/4}$, A. The rim-loaded disk uses a larger fraction of its energy for rotation.

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