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AQA M2 - circular motion

I'm on question 6. I have managed up to part b ii) but I get R=-23mg/30.

Does that mean I've got R pointing the wrong way and if so why? Or, have I mucked up somewhere else?

Thanks.
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Reply 1
Avatar for TeeEm
TeeEm
19
Original post by maggiehodgson
I'm on question 6. I have managed up to part b ii) but I get R=-23mg/30.

Does that mean I've got R pointing the wrong way and if so why? Or, have I mucked up somewhere else?

Thanks.


I did not check the individual workings but this type of motion can sometimes be unintuitive.
If the workings are correct, minus simply implies R is inwards instead of outwards
(you have radially out as positive in your model)
Original post by maggiehodgson


Does that mean I've got R pointing the wrong way and if so why? Or, have I mucked up somewhere else?

Thanks.


At TeeEm said.

Working looks fine.

When you assign a direction to R, you're in effect saying, R is positive in this direction. If it works out negative, then R is actually in the opposite direction.

It's not wrong, rather it's a side effect of the method. You can't know in advance, in this situation, which direction R is acting.
Sorry for the delay.

Thanks for that but should I have known from the question which way R worked?
Reply 4
Avatar for waxwing
waxwing
5
Original post by maggiehodgson
Sorry for the delay.

Thanks for that but should I have known from the question which way R worked?


The direction of R in such questions can sometimes be intuited, and sometimes figured out by hand-wavy arguments (example: at the bottom of the circle it's pretty obvious the reaction force is pointing upwards!), but fundamentally: no, you don't need to know it in advance, and it's acceptable to calculate a negative number and then, if you like, write a note ("or, R has magnitude 23mg/30 towards the centre").

Note the difference between Q6 and Q8 - in Q8 you're travelling over the top of a solid hemisphere - what does that mean about the direction of R?
Original post by waxwing
The direction of R in such questions can sometimes be intuited, and sometimes figured out by hand-wavy arguments (example: at the bottom of the circle it's pretty obvious the reaction force is pointing upwards!), but fundamentally: no, you don't need to know it in advance, and it's acceptable to calculate a negative number and then, if you like, write a note ("or, R has magnitude 23mg/30 towards the centre").

Note the difference between Q6 and Q8 - in Q8 you're travelling over the top of a solid hemisphere - what does that mean about the direction of R?


It will always be away from the centre?
Reply 6
Avatar for waxwing
waxwing
5
Original post by maggiehodgson
It will always be away from the centre?

Yes, which has big implications for that kind of circular motion. When R = 0 it loses contact with the surface (whereas with motion on a bead, say, the R just changes sign at that point).
Reply 7
Avatar for kimisfit
kimisfit
I know the exam answers if you want help. I may be a little out dated though :biggrin:
Reply 8
Avatar for kimisfit
kimisfit
Original post by SwEaTy AsIaN
No BLUUUUUUD U DUMB BLUUD MATHS FOR NERRRRDS


I have fun with nerds, if you get me
Original post by kimisfit
i know the exam answers if you want help. I may be a little out dated though :d

kim is fit ur rite
Original post by SwEaTy AsIaN
No BLUUUUUUD U DUMB BLUUD MATHS FOR NERRRRDS


What!! Are you OK? Not having some sort of episode are you? If so call 999 it'll be quicker.

No idea what you mean but all replies are read as I am probably a nerd.

Do the people who answered you know what you mean?
Original post by maggiehodgson
what!! Are you ok? Not having some sort of episode are you? If so call 999 it'll be quicker.

No idea what you mean but all replies are read as i am probably a nerd.

Do the people who answered you know what you mean?


please learn english before u study maths
Reply 12
Avatar for kimisfit
kimisfit
Original post by maggiehodgson
What!! Are you OK? Not having some sort of episode are you? If so call 999 it'll be quicker.

No idea what you mean but all replies are read as I am probably a nerd.

Do the people who answered you know what you mean?



You mad bro?
Original post by SwEaTy AsIaN
please learn english before u study maths



I will if u will.
Original post by kimisfit
You mad bro?


possibly but probably just old
Original post by maggiehodgson
i will if u will.


i already hav thx unlike u
Original post by SwEaTy AsIaN
i already hav thx unlike u


Where did you learn it? What is your first language? Ah! Perhaps the clue is in your name, Ian. Sounds like a Scottish name to me.

Perhaps year tockin Scoots English in which case a forgive ya, ya cheeky young rascal.
Original post by maggiehodgson
where did you learn it? What is your first language? Ah! Perhaps the clue is in your name, ian. Sounds like a scottish name to me.

Perhaps year tockin scoots english in which case a forgive ya, ya cheeky young rascal.


my first language is english noob and my name is not ian and no im not scotish thank god for that

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