A large number ( n ) of identical beads, each of mass m and radius r are strung on a thin smooth rigid…

A large number (n) of identical beads, each of mass m and radius r are strung on a thin smooth rigid horizontal rod of length L(Lr) and are at rest at random positions. The rod is mounted between two rigid supports (see figure). If one of the beads is now given a speed v, the average force experienced by each support after a long time is (assume all collisions are elastic):

  1. mv2L-nr
  2. mv2L-2nr
  3. mv22L-nr
  4. Zero

Solution

Average time for 1 collision to occur on 1 wall, 
=tavg=average distance travelledspeed=2L-2nrv

tavg=2L-2nrv. Change in momentum in 1 collision =2mv. Average force on each support, favg=2mvtavg=2mv22L-2nrfavg=mv2L-2nr.

Asked in: JEE Main 2015 (11 Apr Online)

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