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# Physics/Direction of friction force

Mikko wrote at 2013-06-15 07:42:56
Rolling can be understood as two motions: the motion of the axis of the roll and the rotation around the axis. First imagine how the roll would glide if it moved

without turning. At the point of contact to the ground there would be a friction

force that is directed against the motion of the roll. This friction makes the roll to turn if there is nothing to prevent that. It also slows down the forward motion if not compensated by some other force. Even when the roll rolls without gliding there is some friction, depending on the quality of the surfaces and the size of the roll. The direction of the friction force is still against the motion of the roll. But when the roll is forced to turn faster than it would in non-gliding rolling the friction force is in forward direction, the contacting part of the roll moves in the opposite direction and the friction force is against THAT

direction, i.e., in the direction of the motion of the axis.

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