# What's the direction of the static friction force in this free body diagram? [closed]

I know that static friction is a force that opposes any force of motion and in this case we have two:

1. the parallel component of weight force,

2. tension force.

So the static friction can oppose the parallel component of weight force or the tension force.

So my question is:

Which direction will the static friction force go in?

• Hint: Static friction will oppose the net external parallel force acting on the block. – Bob D Nov 5 '19 at 17:35
• It all depends on which way the block is moving. The frictional force will be in the opposite direction to the motion of the block so as to try and reduce the relative motion between the block and the slope. – Farcher Nov 5 '19 at 17:51
• You don't automatically know without working the problem. But that often doesn't matter. Pick a direction and go to town. If you picked right you'll compute a positive value for the frictional force; if you chose wrong a negative one. The sign of the result tells you how good your intuition was. – dmckee --- ex-moderator kitten Nov 5 '19 at 18:16

The friction force opposes the direction of the movement, not necessarily the weight's parallel component.

If the tension force is greater than the weight's parallel force the object will want to slide up the ramp and the friction force will be pointing opposite of the tension force along the ramp. The static friction force will try to hold the object stationary while growing up from zero gradually to the level needed fore dislodging the object. Then it will drop down to the kinetic friction. So the static friction is a transient force just before the object starts to move.

If the tension force is smaller than the weight's parallel component the object will want to move down and the friction force will be parallel to the tension. And same process as mentioned above will happen in reverse direction.