Q. 294.6( 8 Votes )

# Explain why friction is necessary to make the disc in Fig. 7.41 roll in the direction indicated.

A. Give the direction of frictional force at B, and the sense of frictional torque, before perfect rolling begins.

B. What is the force of friction after perfect rolling begins?

Answer :

For rotation of the disc torque is required. If friction is absent at B due to tangential force (Torque) at A disc will rotate in the same place but won’t move forward. So, friction is necessary to make the disc to move forward.

A. Friction opposes motion. So, frictional force is opposite to the direction of linear velocity at point B, in rightward direction. Sense of frictional torque, before perfect rolling begins is perpendicular to plane of disc in the outward direction.

B. Perfect rolling means the linear velocity at point B is zero. So, frictional force is also zero.

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PREVIOUSA disc rotating about its axis with angular speed ω0 is placed lightly (without any translational push) on a perfectly frictionless table. The radius of the disc is R. What are the linear velocities of the points A, B and C on the disc shown in Fig. 7.41? Will the disc roll in the direction indicated?NEXTA solid disc and a ring, both of radius 10 cm are placed on a horizontal table simultaneously, with initial angular speed equal to 10 π rad/s. Which of the two will start to roll earlier? The co-efficient of kinetic friction is μk= 0.2.

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