What direction does friction point in a diagram of a spring?

[Danya314]

Homework Statement

A mass is at rest on a spring on an inclined plane. Which way would I draw the force of friction on a force diagram?

Homework Equations

Fnet=0
F=-kx

The Attempt at a Solution

It could point either up the incline, or down the incline.
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[TSny]

It could point either up the incline, or down the incline.

Yes, that's correct.

 

[Danya314]

So, which is it?

 

[Dick]

So, which is it?

The force of friction always points opposite the direction the object is moving.

 

[SammyS]

The force of friction always points opposite the direction the object is moving.

or in a direction to oppose motion. (It may be at rest.)

 

[haruspex]

The force of friction always points opposite the direction the object is moving.

No, it opposes relative motion of the surfaces in contact. I know that's what you meant, but expressed the way you had it can lead to confusion, e.g. when a car is going uphill.
https://www.physicsforums.com/insights/frequently-made-errors-mechanics-friction/

 

[Dick]

No, it opposes relative motion of the surfaces in contact. I know that's what you meant, but expressed the way you had it can lead to confusion, e.g. when a car is going uphill.
https://www.physicsforums.com/insights/frequently-made-errors-mechanics-friction/

Ok, you guys make good points. I yield. I was simplicitic.

 

[Danya314]

So in this case, would it point down the incline?

 

[haruspex]

So in this case, would it point down the incline?

Suppose there were no friction. Which way would the block move? Is there enough information to decide?

 

[TSny]

So, which is it?

If a block, attached to a spring, is at rest on an inclined plane and that's all the information you have, then there is not enough information to deduce the direction of the friction force. It could be up the incline or down the incline, depending on the amount by which the spring is stretched or compressed. That's why I said your statement in the "attempt at a solution" section was correct.

 

[Danya314]

If friction was ignored, then I would assume the the net force would pull the block down the incline and strech the spring.

 

[haruspex]

If friction was ignored, then I would assume the the net force would pull the block down the incline and strech the spring.

You wrote that it was "on" the spring. Is it above the spring or below it on the slope?
Either way, as TSny keeps telling you, there is not enough information to decide. The spring may be under tension or under compression.