Rotation 3 Question 25

31. A small sphere rolls down without slipping from the top of a track in a vertical plane. The track has an elevated section and a horizontal part. The

horizontal part is 1.0 m above the ground level and the top of the track is 2.6m above the ground. Find the distance on the ground with respect to the point B (which is vertically below the end of the track as shown in figure) where the sphere lands. During its flight as a projectile, does the sphere continue to rotate about its centre of mass? Explain.

(1987,7 M)

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Answer:

Correct Answer: 31. 2.13m, yes

Solution:

  1. h=2.61.0=1.6m

During pure rolling mechanical energy remains conserved

So, at bottom of track total kinetic energy of sphere will be mgh.

The ratio of KRKT=25

 or KT=57mgh=12mv2v=107gh=107×9.8×1.6=4.73m/s

In projectile motion

Time to fall to ground =2×19.8=0.45s

The desired distance BC=vt=2.13m

In air, during its flight as a projectile only mg is acting on the sphere which passes through its centre of mass. Therefore, net torque about centre of mass is zero or angular velocity will remain constant.



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