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Question

How does friction make an ball roll

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Solution

The ball rolls because: thefriction present between the surface pushes the atoms/particles in contact with surface or ground backwards, hence with centre of mass moving in a direction, the ball'sbottom surface(ie. The surface in contact) moves in opposite direction or backwards, causing the ball to roll.


If you consider a ball intially at rest on a frictionless surface, if a force is exerted through the centre of mass of the ball it will slide across the surface with no rotation, there will only be translational motion.

If you consider the case where there is friction, if the force is again applied to a stationary ball the frictional force will act in the opposite direction to the force but at the edge of the ball that rests on the ground. This friction applies a torque to the ball which starts the rotation. So static friction is infact necessary for a ball to begin rolling.

The condition for rolling at speed vvis that the angular velocity of the ball is given by

v=ωr

from this the top of the ball will move at speed 2v the centre of mass of the ball will move at vv and the bottom edge of the ball will instantaneously be at rest. So as the edge touching the ground is stationary it experiences no friction.

So friction is necessary for a ball to start rolling but once the rolling condition has been met the ball experiences no friction.


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