Explain why friction is necessary to make the disc in roll in the dire

1.	Explain why friction is necessary to make the disc in roll in the direction indicated.Give the direction of the frictional force at B, and the sense of frictional torque, before perfect rolling begins.What is the force of friction after perfect rolling begins?
Answer» Friction is necessary for rolling. 1. Frictional force at B opposes velocity of B. So, the frictional force is in the same direction as the arrow. The sense of frictional torque is such as to oppose angular motion, \(\vec {ω_0}\) and \(\vec {τ_0}\) are both normal to the paper, the first into the paper and the second coming out of the paper. 2. Frictional force decreases the velocity of the point of contact B. Perfect rolling ensures when this velocity is zero. Once this is so, the force of friction is zero.

Explain why friction is necessary to make the disc in roll in the direction indicated.Give the direction of the frictional force at B, and the sense of frictional torque, before perfect rolling begins.What is the force of friction after perfect rolling begins?

Answer»

Friction is necessary for rolling.

1. Frictional force at B opposes velocity of B. So, the frictional force is in the same direction as the arrow. The sense of frictional torque is such as to oppose angular motion, \(\vec {ω_0}\) and \(\vec {τ_0}\) are both normal to the paper, the first into the paper and the second coming out of the paper.

2. Frictional force decreases the velocity of the point of contact B. Perfect rolling ensures when this velocity is zero. Once this is so, the force of friction is zero.

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