We always encounter this expression: "Friction always opposes the motion". But what does it mean exactly? For example when we push a heavy box and it does not move we consider the friction opposite to the applied force. The second case when the box moves. In this case if we show the displacement by vector X the friction will be a vector opposite to X. Now consider a mass on an rotating horizontal disc. In this case we consider the friction equal to mrw^2 which w is the angular velocity and we consider the friction towards the center of rotation. How can we interpret the expression "friction is opposite to the motion". In the first case we dont have any motion at all. At the third case the displacement vector is different to the friction.
What is the quantitative meaning of "motion"? Can we consider it the displacement vector?
Can you explain this?
The Attempt at a Solution