A car drives with its tires rolling freely. Is the friction between the tires and the road (a) kinetic or (b) static?

 

C O N C E P T U A L  C H E C K  P O I N T

F R I C T I O N  F O R  R O L L I N G   

T I R E S

A car drives with its tires rolling freely. Is the friction between the tires and the road
(a) kinetic or (b) static?

R E A S O N I N G   A N D     D I S C U S S I O N

A reasonable-sounding answer is that because the car is moving, the friction between its tires and the road must be kinetic friction—but this is not the case.

Actually, the friction is static because the bottom of the tire is in static contact with the road. To understand this, watch your feet as you walk. Even though you are moving, each foot is in static contact with the ground once you step down on it. 

Your foot doesn’t move again until you lift it up and move it forward for the next step. A tire can be thought of as a succession of feet arranged in a circle, each of which is momentarily in static contact with the ground.

A N S W E R

(b) The friction between the tires and the road is static friction.

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