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

OP

Original post by Lau14

Recently I've been spending more around the 25-35 on it, but I don't tend to worry too much if I get stuck on things - my exam technique tends to be very time effective. How long do you usually spend?

I usually spend around 40-45 minutes on it. Wasn't sure if that was too much/little. Was gonna try and split it up based on its weighting (ie: spend longer on section B) but Idk

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Reply 1362

Original post by CD223

Just that I wasn't sure if deviations in gravitational field strengths would really come into circular motion, unless considering a satellite in orbit.

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They usually mix things together and would want us to work them out.
Did you know that the inverse square proportionality of gravitational force can be worked out with centripetal force and Kepler's third law?

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Reply 1363

OP

Original post by Mehrdad jafari

They usually mix things together and would want us to work them out.
Did you know that the inverse square proportionality of gravitational force can be worked out with centripetal force and Kepler's third law?

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Yeah by equating

\frac{GMm}{r^2}

with

\frac{mv^2}{r}

proving that

r^{3} \alpha T^{2}

, right?

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Reply 1364

Original post by CD223

Yeah by equating

\frac{GMm}{r^2}

with

\frac{mv^2}{r}

proving that

r^{3} \alpha T^{2}

, right?

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No i meant how do we actually know that the gravitational force is inversely proportional to distance square. It caught my attention when i read it on a textbook.
How do you make pictures like how you've done so i can do the same?

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Reply 1365

OP

Original post by Mehrdad jafari

No i meant how do we actually know that the gravitational force is inversely proportional to distance square. It caught my attention when i read it on a textbook.
How do you make pictures like how you've done so i can do the same?

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Henry Cavendish used a torsion balance to measure the mass of the earth, which is an experiment which used this principle. If you google it you'll probably find an explanation as to why g varies with the inverse of

r^2

.

It's called LaTeX. It's a kind of code used for writing and formatting maths equations online. If you search for it online you'll see how to use it. I think there's a student room page too.

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Reply 1366

Lau14

11

Original post by CD223

I usually spend around 40-45 minutes on it. Wasn't sure if that was too much/little. Was gonna try and split it up based on its weighting (ie: spend longer on section B) but Idk

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45 minutes is the recommended time so I'd think you'll be alright unless you're really short for time on the written half?

Reply 1367

Original post by CD223

Henry Cavendish used a torsion balance to measure the mass of the earth, which is an experiment which used this principle. If you google it you'll probably find an explanation as to why g varies with the inverse of

r^2

.

It's called LaTeX. It's a kind of code used for writing and formatting maths equations online. If you search for it online you'll see how to use it. I think there's a student room page too.

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Let me know what you think:

F \alpha \frac{v^2}{r}

and

v^{2} \alpha \frac{r^2}{T^2}

, but

T^{2} \alpha r^{3}

so

v^{2} \alpha \frac{1}{r}

, hence

F \alpha \frac{1}{r^2}

Reply 1368

ubisoft

3

You don't need to know any of that lmao. You just need to know Newton's formula which says F is inversely proportional to r^2.

Reply 1369

Original post by ubisoft

You don't need to know any of that lmao. You just need to know Newton's formula which says F is inversely proportional to r^2.

That's true, it's not required in the specification. But if you know why F is inversely proportional to distance square, and that Newton didn't just assume that F is inversely proportional to distance square because the equation would look better with that, then you will never forget that F is inversely proportional to distance square.

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Reply 1370

OP

Original post by ubisoft

You don't need to know any of that lmao. You just need to know Newton's formula which says F is inversely proportional to r^2.

Original post by Mehrdad jafari

Let me know what you think:

F \alpha \frac{v^2}{r}

and

v^{2} \alpha \frac{r^2}{T^2}

, but

T^{2} \alpha r^{3}

so

v^{2} \alpha \frac{1}{r}

, hence

F \alpha \frac{1}{r^2}

You don't need to know quite that much detail but there was a question in the June 2014 paper that required you to work out the time period of a satellite. I found that was made easier with knowledge of the equation:

T^{2}=\frac{4\pi^{2}r^{3}}{GM}

Reply 1371

Original post by CD223

You don't need to know quite that much detail but there was a question in the June 2014 paper that required you to work out the time period of a satellite. I found that was made easier with knowledge of the equation:

T^{2}=\frac{4\pi^{2}r^{3}}{GM}

Yeah, you're right, but this is just to give an insight into how scientific research is carried out. Did you know that many students lose interest in science just because they are not told these kind of things? If I'm studying Newton's law of gravity and I'm not told how the equation was worked out, then for me scientific discovery is impenetrable, it would make me feel as if I'm not good enough to think about my own ideas. This doesn't have to be science, it's true in everything.

You need to learn the rules of the game, and then you can play better than anyone else.
Albert Einstein

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Reply 1372

OP

Original post by Mehrdad jafari

Yeah, you're right, but this is just to give an insight into how scientific research is carried out. Did you know that many students lose interest in science just because they are not told these kind of things? If I'm studying Newton's law of gravity and I'm not told how the equation was worked out, then for me scientific discovery is impenetrable, it would make me feel as if I'm not good enough to think about my own ideas. This doesn't have to be science, it's true in everything.

You need to learn the rules of the game, and then you can play better than anyone else.
Albert Einstein

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Lol very true. Learning subjects at A level sometimes takes the fun out of them.

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Reply 1373

Original post by CD223

Lol very true. Learning subjects at A level sometimes takes the fun out of them.

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Lol, there we go!

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Reply 1374

JJBinn

2

Well had the EMPA A2 written section today, all I can say is good luck to anybody who still has it to do, absolutely solid

Reply 1375

Original post by JJBinn

Well had the EMPA A2 written section today, all I can say is good luck to anybody who still has it to do, absolutely solid

Lol, i know. But some of the questions were completely outlandish. If i was a teacher i would strongly criticise them

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Reply 1376

JJBinn

2

Original post by Mehrdad jafari

Lol, i know. But some of the questions were completely outlandish. If i was a teacher i would strongly criticise them

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Yeah haha, discussing the answers after with other people and everyone got different answers for everything :biggrin:

Reply 1377

Original post by JJBinn

Yeah haha, discussing the answers after with other people and everyone got different answers for everything :biggrin:

Mark scheme is coming soon so we can be sure of criticising them, lol

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Reply 1378

JJBinn

2

Original post by Mehrdad jafari

Mark scheme is coming soon so we can be sure of criticising them, lol

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What do you mean?

Reply 1379