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

Usually I'm okay with these questions, but this one has stumped me:

Jan 2013 - any takers?




Posted from TSR Mobile

It was meant to be something like this even thought i used my own results which give me an inaccurate mass of the DVD. X is the mass of the DVD



Posted from TSR Mobile

using M = Mass of moon, m = mass of earth, R = radius of moon and r = radius of earth

Gm/(r^2) = 6 GM/(R^2) (gravitational field strength)

0.6 m/(4/3*pi*r^3) = M/(4/3*pi*R^3) (density)

We can then cancel out to get

m/r^3 = 0.6 M/R^3
m/r^2 = 6 M/R^2

Dividing the second set by the first set gives

r = 6/0.6R
r/R = 10

How about this?

First pair of equations are given ratios.
Second pair of equations are the relevant equations, replacing the other constants with k and rearranging for k to form the next equations nicely.
The last pair of equations are from making the k equations equal which is true since the original ratios are substituted in at the same time.

Ah but it says the answer is 3.6 :/


Posted from TSR Mobile

If you take my expression and your expression with a mass of 200g you get the same answers.

In my answer I used both the 3.1% increase and 3.3% increase which both ended up with answers ~13g.

The 3.1% equation produced:
12.5922g

The 3.3% equation produced:
13.4178g

Averaging the two results gives a mass of the DVD of 13.005g.


Posted from TSR Mobile

Oh dear >< again I don't know what I'm doing wrong...Although JayCS' method seems unassailable.

Oh right. Then yeah. I though of doing that too in the exam but i scrabbled so much that i ended up having no space left for the answer, lol


Posted from TSR Mobile

Ah I was in a similar position Hahha!

Be interesting to see how the MS does it if there were multiple methods.


Posted from TSR Mobile

using M = Mass of moon, m = mass of earth, R = radius of moon and r = radius of earth

Gm/(r^2) = 6 GM/(R^2) (gravitational field strength)

0.6 m/(4/3*pi*r^3) = M/(4/3*pi*R^3) (density)

We can then cancel out to get

m/r^3 = 0.6 M/R^3
m/r^2 = 6 M/R^2

Dividing the second set by the first set gives

r = 6/0.6R
r/R = 10

Hi there.

Your method works; I think you just made a mistake on the first equation you formed from cancelling out.

I get that it is m/r^3=5M/3R^3 i.e. 1/0.6 not 0.6 on the RHS.

The rest worked out from there.

Hi there.

Your method works; I think you just made a mistake on the first equation you formed from cancelling out.

I get that it is m/r^3=5M/3R^3 i.e. 1/0.6 not 0.6 on the RHS.

The rest worked out from there.

Do you use the same approach for all ratio questions?

Hey.

I use this format for questions that require more than one ratio to be used -- in this case, for gravitational field strength and density.

For other ratio questions, I just form one ratio equation such as below.



I don't use either method that often though since these questions don't seem to be that common; I end up using proportions for multiple choice most of the time.

CD223,

Here's the set of questions =D Some are past questions, some are ones they think are likely to come up.

Cool! thanks. How are you doing on past papers?

PHYA4 papers are good to do since PHYA4 was the first unit we learned. I suspect it's the same for most people. I've only just starting doing PHYA5 papers though since there aren't that many to do.

Yourself?