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# AQA Physics PHYA4 - Thursday 11th June 2015 [Exam Discussion Thread] watch

1. (Original post by Jed-Singh)
ah, so is Resultant Force (R) the opposite of T? ie the negative form of those equations is R?
Oh sorry, I mistook your (R) to mean the reaction force.

The resultant force is the centripetal force, and is always directed towards the centre of the circle.

The centripetal force IS the resultant force, and as such there's not a PHYSICAL force labelled as the "centripetal force". It is more a case of the centripetal/resultant force being the net effect of all other forces acting on the body to keep it moving in a circular path.

In vertical circles, for example on a fairground Ferris wheel, generally the upwards and downwards forces are the normal reaction force (labelled N or R) and the weight (mg - which always acts vertically downwards).

At the top, the reaction force acts upwards and the weight acts downwards, so the resultant force, mv^2/r = mg - N because the weight is pointing towards the centre of the circle.

At the bottom the reaction force still acts upwards and the weight acts downwards, but this time the reaction force acts upwards towards the centre, so the resultant force, mv^2/r = N - mg because the reaction is pointing towards the centre of the circle.

Does this make sense?

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2. (Original post by CD223)
Oh sorry, I mistook your (R) to mean the reaction force.

The resultant force is the centripetal force, and is always directed towards the centre of the circle.

The centripetal force IS the resistance force, and as such there's not a PHYSICAL force labelled as the "centripetal force". It is more a case of the centripetal/resultant force being the net effect of all other forces acting on the body to keep it moving in a circular path.

In vertical circles, for example on a fairground Ferris wheel, generally the upwards and downwards forces are the normal reaction force (labelled N or R) and the weight (mg - which always acts vertically downwards).

At the top, the reaction force acts upwards and the weight acts downwards, so the resultant force, mv^2/r = mg - N because the weight is pointing towards the centre of the circle.

At the bottom the reaction force still acts upwards and the weight acts downwards, but this time the reaction force acts upwards towards the centre, so the resultant force, mv^2/r = N - mg because the reaction is pointing towards the centre of the circle.

Does this make sense?

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Wow you really know your physics! thanks so much, that makes much more sense.
The question that had me stuck in the first place was June 2011 1)aii on section B where it asked to write and equation relating contact force, R, with mvrg for a parcel on the floor of a van travelling over the highest point of a bridge. So that would be the force acting upwards right? Hence the equation would be mg-mv^2/r ?
3. (Original post by Jed-Singh)
Wow you really know your physics! thanks so much, that makes much more sense.
The question that had me stuck in the first place was June 2011 1)aii on section B where it asked to write and equation relating contact force, R, with mvrg for a parcel on the floor of a van travelling over the highest point of a bridge. So that would be the force acting upwards right? Hence the equation would be mg-mv^2/r ?
Ah thank you!

Can I just clarify something though, I accidentally said "The centripetal force IS the resistive force" when I meant "The centripetal force IS the resultant force"

Regarding the question, if the contact force acts upwards and weight acts downwards, and the centre of the "circle" is below the van, then:
mv^2/r = mg-R
So yeah, R = mg - mv^2/r

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4. Unit 4 isn't that bad actually, it's the empa that's worrying
5. (Original post by Johff)
Unit 4 isn't that bad actually, it's the empa that's worrying
I find physics as a whole rather worrying where it comes to exams. It seems to be the most unpredictable of my subjects.

The EMPA isn't fun. Last year I got a C so I don't have a good track record. How are you preparing for section B? (Without giving away details of section A for those that haven't done it).

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6. [QUOTE=CD223;55183161]I find physics as a whole rather worrying where it comes to exams. It seems to be the most unpredictable of my subjects.

The EMPA isn't fun. Last year I got a C so I don't have a good track record. How are you preparing for section B? (Without giving away details of section A for those that haven't done it).

Looking at past papers, with similar tasks. Also looking at uncertainties, errors etc
7. (Original post by Johff)
Looking at past papers, with similar tasks. Also looking at uncertainties, errors etc
Oh right!
That makes sense. Lau14 prepared a great guide to the A2 EMPA. See the first post of this thread and click on the spoiler button in the relevant resources section of that post to find it

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8. [QUOTE=CD223;55183497]Oh right!
That makes sense. Lau14 prepared a great guide to the A2 EMPA. See the first post of this thread and click on the spoiler button in the relevant resources section of that post to find it

Cool, will have a look in a bit.
9. (Original post by Johff)
Cool, will have a look in a bit.
Sure. Good luck for the 7th.

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10. (Original post by CD223)
Sure. Good luck for the 7th.

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Good luck to you to!

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11. (Original post by Johff)
Good luck to you to!

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Thanks! Let us know how you find it! I'll have done it two days earlier haha.

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12. (Original post by CD223)

Just relax and take a look through the Nelson Thornes AQA textbook if you have it. It's useful for both sections

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Thanks. I'll take your advice. My second practical test is next tuesday.
13. (Original post by Samphosrattana)
Thanks. I'll take your advice. My second practical test is next tuesday.
Oh nice! Do you know when your section B is? Mine is on 5th May

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14. (Original post by CD223)
Oh nice! Do you know when your section B is? Mine is on 5th May

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Mine is on 7th May.
15. (Original post by Samphosrattana)
Mine is on 7th May.
Ah so many people have theirs on 7th May! Mines 5th

Kinda worried as I've done no prep so far, but it's really hard to...

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16. (Original post by Conort20)
could you possibly upload a link to the unit 5 paper and mark scheme? Thanks
If it's uploaded it's probably going to be on the PHYA5 thread. I'll add the 2014 papers to the first post in each thread soon.

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17. Let's say I get U for my overall practical test. What grade must I need to get for written test to obtain B, C, or D grade overall? I need to get B overall for physics to get into ediburgh university, but it seems like my chance is slim.😔 I'm going to retake all physics units this summer.

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18. (Original post by Samphosrattana)
Let's say I get U for my overall practical test. What grade must I need to get for written test to obtain B, C, or D grade overall? I need to get B overall for physics to get into ediburgh university, but it seems like my chance is slim.😔 I'm going to retake all physics units this summer.

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It's hard to say overall without knowing last year's results, but based on this year, it's 210/300 for a B, 180/300 for a C and 150/300 for a D.

This practical unit is worth 60 UMS, so it's perfectly feasible for you to get enough UMS in both theory papers to amount to those grades for this year overall, but it's hard for me to understand what that would be overall without knowing last year's results.

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19. (Original post by CD223)
It's hard to say overall without knowing last year's results, but based on this year, it's 210/300 for a B, 180/300 for a C and 150/300 for a D.

This practical unit is worth 60 UMS, so it's perfectly feasible for you to get enough UMS in both theory papers to amount to those grades for this year overall, but it's hard for me to understand what that would be overall without knowing last year's results.

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Thanks for this useful information. Frankly speaking, I got U for unit 3 empa last year and C for unit 2. However, I'm going to retake them. So basically, I'm going to sit the whole physics exams this year.
20. (Original post by Samphosrattana)
Thanks for this useful information. Frankly speaking, I got U for unit 3 empa last year and C for unit 2. However, I'm going to retake them. So basically, I'm going to sit the whole physics exams this year.
Oh right, well it's 70% UMS overall for a B, 60% for a C, etc. The two years are out of 600 UMS total. Each of the 4 exams are weighted as 120 UMS and both practical units are worth 60 UMS.

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