I was doing some problems, and this thought occurred to me. I've attached the question with the diagrams. Appreciate the help!
That's a useful rule for static situations, not when sliding occurs.coreankim said:Friction always points in the direction opposite the net force (excluding friction): this makes intuitive sense.
I'll assume you are talking about a block sliding up an incline (let's not worry about wheels, etc.). So we are talking about kinetic friction, not static. It's easy to find the direction of kinetic friction: As the block slides up, the friction points down; as it slides down, the friction points up.However, this is not always true. For example, let's say that a car that was going up on an inclined plane suddenly breaks. In this case, the only force other than friction that acts on the car is weight, which points down the plane. However, because the car still needs to go some amount up the plane before it stops, friction also points down the plane. The direction of friction and the net force excluding friction point in the same direction.