Why is it said that friction always acts in the direction opposite to the relative motion at the point of contact with the surface?
When a sphere is rolling down an incline for example, the friction acts upwards in order to create a torque which increases the sphere's angular velocity to prevent rolling without slipping. The thing is, the relative motion between the point of contact and the sphere is also upwards.
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1 Answer
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When the sphere is rolling without slipping there is no relative motion between the sphere and the surface at the point of contact.
The ball "wants" to slip, such that there will be a relative motion between the surfaces however what prevents this slipping is the static friction force, acting against the potential direction of motion.
