Direction of kinetic vs static friction

[deleted388502]

Okay! So I know that friction always opposes the direction of motion. So if you have like a box on an inclined plane, friction will oppose the direction of gravity. I got an interesting problem today that I got wrong that made me want to clear something up:

I think I misunderstood the question when I first did it because I had friction pointing the other way, assuming that it was still opposing the force of gravity. My question is how I should have known to draw friction in the direction of gravity when I drew the free body diagram?

The answer choices tipped me off for sure, but I haven't come across question like this recently and was wondering if someone could explain to me how I would know when friction would be acting in the same direction as direction of motion from the wording of a question.
If something has overcome its coefficient of static friction, and has kinetic friction, does it therefore have kinetic friction acting in the same direction of motion then?

Pretty simple question, but I just wanted to make sure I had this down.

Thanks!

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[Cawolf]

Okay! So I know that friction always opposes the direction of motion.

Actually, friction opposes motion - and on an inclined plane, gravity and motion are in different directions (as a technicality ).

I solved it the long way and took a pic. Here is my sketch - maybe it will help.

 

[Cawolf]

In this case - friction is opposing motion as the block is moving up the incline.

 

[deleted388502]

Actually, friction opposes motion - and on an inclined plane, gravity and motion are in different directions (as a technicality ).

I solved it the long way and took a pic. Here is my sketch - maybe it will help.

Hey! Thanks for the picture. I definitely thought that friction always opposed motion which is why the picture they provided in that solution confused the heck out of me.
Maybe I'm thick, but I'm still a bit confused by your picture. Why do you have friction pointing down the inclined plane in your initial picture? Shouldn't it be pointing the way Fd is? I think I'm just confused about which way friction should be pointing on inclined planes, perhaps, and also what this "driving" force in this problem means?

Thanks for taking the time to help me out!

 

[deleted388502]

In this case - friction is opposing motion as the block is moving up the incline.

AHHH. Okay. So in this case, the "driving" force is taking precedence as the motion the friction is opposing??

 

[Cawolf]

Their picture is correct as they state it is moving up the plane - so friction points down it.

The "Driving Force" is some force moving the block up the plane against the direction of gravity.

If you look at my picture, it is a free body diagram that shows all forces acting on the block. They state that the block is accelerating up the plane at 2 m/s^2 so by F = ma we can solve for the force that is being applied upwards at 30 degrees to accelerate the block up the plane.

 

[Cawolf]

It is important to determine the direction of motion and then you know that kinetic friction is anti-parallel.

If it is static friction, it will oppose the forces that seek to move the object.

 

[deleted388502]

I think the point I was missing is that the force friction was opposing was the driving force and not gravity. Makes sense now.

Thank you!