# 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