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M1 help please (again)

http://pmt.physicsandmathstutor.com/download/Maths/A-level/M1/Solutionbank-Heinemann/M1%20Chapter%204.pdf

Exercise D, Question 23

Part B

So the mark scheme finds the normal reaction by resolving vertically. However I resolved horizontally and used F = Rμ (idk if this is right) to find R. As you can guess I got the wrong answer, but when I resolved vertically I got the right answer. So are there circumstances where you have to resolve vertically or have I just made a stupid mistake?
Original post by AxSirlotl
http://pmt.physicsandmathstutor.com/download/Maths/A-level/M1/Solutionbank-Heinemann/M1%20Chapter%204.pdf

Exercise D, Question 23

Part B

So the mark scheme finds the normal reaction by resolving vertically. However I resolved horizontally and used F = Rμ (idk if this is right) to find R. As you can guess I got the wrong answer, but when I resolved vertically I got the right answer. So are there circumstances where you have to resolve vertically or have I just made a stupid mistake?


But surely you can see that it's moving horizontally so F≠μRF \neq \mu R.
In other words, it is not in equilibrium horizontally.
Reply 2
Avatar for Pangol
Pangol
15
Original post by RDKGames
But surely you can see that it's moving horizontally so F≠μRF \neq \mu R.
In other words, it is not in equilibrium horizontally.


I agree that it's not in equilibrium (which is why resolving horizontally hasn't worked), but surely F is equal to mu R since the frictional force is now as large as it could possibly be?
Reply 3
Avatar for AxSirlotl
AxSirlotl
OP
18
Original post by RDKGames
But surely you can see that it's moving horizontally so F≠μRF \neq \mu R.
In other words, it is not in equilibrium horizontally.


I didn't pick up on that so yes that makes sense now, thanks
Reply 4
Avatar for AxSirlotl
AxSirlotl
OP
18
Original post by Pangol
I agree that it's not in equilibrium (which is why resolving horizontally hasn't worked), but surely F is equal to mu R since the frictional force is now as large as it could possibly be?


I was using F = MA to find the normal reaction and I set F = 0 as I didn't see it was moving, so F wouldn't equal 0 but F (as in frictional force, not total force) would equal R mu (I think).
Original post by Pangol
I agree that it's not in equilibrium (which is why resolving horizontally hasn't worked), but surely F is equal to mu R since the frictional force is now as large as it could possibly be?


FF here is the total horizontal force going to the right.

F=μRF=\mu R would imply that it is on the verge of moving and the process is limiting.

But clearly it is already moving so F>μRF > \mu R.

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