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Hard friction question

Hi, I'm struggling to answer this question as this isn't like the only question where you need to find the angle of inclination, I would greatly appreciate any help.

Thanks in advance.
Screenshot 2022-10-17 181655.png
(edited 1 year ago)
Original post by Kingemperor07
Hi, I'm struggling to answer this question as this isn't like the only question where you need to find the angle of inclination, I would greatly appreciate any help.

Thanks in advance.
Screenshot 2022-10-17 181655.png


Any thoughts? How would you normally proceed with a box on a slope type question? What do the two possible values of alpha relate to? What have you been able to do so far?
Original post by ghostwalker
Any thoughts? How would you normally proceed with a box on a slope type question? What do the two possible values of alpha relate to? What have you been able to do so far?

Normally I would get the forces up and down the slope and equate them since they're in limiting equillibrium, I then re-arrange and get tan(alpha) = some expression, but this not only gives the wrong answer, but there should be another answer that i'm still missing.
the two angle values would correspond to whether the box is about to move

i) up the slope

ii) down the slope
Original post by Kingemperor07
Normally I would get the forces up and down the slope and equate them since they're in limiting equillibrium, I then re-arrange and get tan(alpha) = some expression, but this not only gives the wrong answer, but there should be another answer that i'm still missing.


Resolving up/down the plane when in equilibrium will work.

Don't forget there are two possible limiting equilibria, on the point of sliding down the slope, and ....

Post working if you'd like someone to check it.
(edited 1 year ago)
Original post by ghostwalker
Resolving up/down the plane when in equilibrium will work.

Don't forget there are two possible limiting equilibria, on the point of sliding down the slope, and ....

Post working if you'd like someone to check it.

I managed to figure it out once you said that limiting equilibria could be either up or down the slope, thanks :smile:
Original post by Kingemperor07
Hi, I'm struggling to answer this question as this isn't like the only question where you need to find the angle of inclination, I would greatly appreciate any help.

Thanks in advance.
Screenshot 2022-10-17 181655.png

I know this is old but do you still have the answers or know where the question is from?
Reply 7
The key point here, that leads to two answers is that the 10N could be just enough force to stop the box sliding down the slope or it could be enough to almost start the box sliding up the slope.

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