Unit 5 Physics Edexcel A2 and Edexcel IAL

Original post by 16characterlimit

Does anyone have any tips/methods on getting the marks for the written questions. Half the time the mark scheme wants things the question doesn't ask for and the other half they are incredibly nitpicky and I find myself writing the entire do not credit/ignore/reject section.

- I find it is best to state the obvious at first, because sometimes you can easily pick up marks there even if it seems too easy.
- Write in bullet points - you do not need to write in prose as this may end up becoming more confusing - and make sure your ideas are coherent and link together. Showing your train of thought will make it easier for you and the examiner and you may end up getting to those points that seem irrelevant at first.
- If there is a mathematical relationship that you can describe with an equation, always state the variables and explain what it shows. You cannot assume the examiner knows everything and could also pick up another mark.
- The mark schemes are nitpicky, but remember for QWC questions in particular, they will either accept equivalents or have more points than necessary. It would be good in this case to write one or two extra points than the question is worth so you cover all bases.
- Stay calm - the worst thing you can do is rush the written questions, panic and then write down everything that you know. I tend to spend a little more time gathering my thoughts on these questions. If you feel like you haven't gained all the marks, make sure you leave enough room just in case you remember something at the end and need to add it in. Chances are, and this happens to me a lot, you will go through other questions and then remember a vital point or realise that you have made an incorrect point.

As for the whole do not credit/reject stuff, it is a case of reading the question properly and understanding the situation they are describing. It's also worthwhile looking through past mark schemes because they sometimes include and omit different points for what is effectively the same question.

Reply 81

Hi! I just wanted to ask that if a system is resonating at it's natural frequency and if we want to damp the system by using a critical damper, to what extent would it affect the vibration?

Reply 82

Original post by sabahshahed294

Hi! I just wanted to ask that if a system is resonating at it's natural frequency and if we want to damp the system by using a critical damper, to what extent would it affect the vibration?

Critical damping allows the system to return to its equilibrium position in the quickest amount of time without any further oscillations, so there would effectively be no vibration after you add the damper. If you over damp the system, it will also return to its equilibrium position without further oscillation, however it will be over a longer period of time.

Reply 83

Original post by PhysicsIP2016

I didn't get your last point actually. You said that it should return to the equilibrium position in the shortest possible time so how come it's going to be over a long period of time? By any chance, do you mean it's going to take a longer time for the system to return to equilibrium than a normal system that's damped? or the time period is longer?

Reply 84

Original post by sabahshahed294

Sorry, I was making two separate points here to differentiate between critical damping and overdamping - they are both essentially the same thing as the oscillations do not continue after they return to equilibrium, however Critical damping occurs in the shortest possible time while overdamping occurs over a longer period of time.

(edited 7 years ago)

Reply 85

@sabahshahed294 This diagram should help

Reply 86

Original post by PhysicsIP2016

@sabahshahed294 This diagram should help

Oh Alright! Thank you very much! :smile:

Reply 87

Hi! I wanted to ask about two MCQs which I got wrong while solving the paper actually so I just wanted to know exactly where I went wrong and why.
https://a4942901ab27cf2817f7a4f7497d95e45091f796.googledrive.com/host/0B1ZiqBksUHNYTU9SWFFTVjI5T0k/June%202013%20(R)%20QP%20-%20Unit%205%20Edexcel%20Physics.pdf
I went wrong in 2 and 9 so just wanted to know:-
In question 2, why is the answer C instead of D? What does the random nature of decay of a radioactive substance got to do with the the probability of decaying in a fixed time interval?
In question 9, I'm totally sort of blank and kinda weak in this part of the spec so just wanted to know how did you even do this?

Reply 88

Original post by sabahshahed294

I can't help with question 2 - but for 9:
Gravitational field strenth follows an inverse square law where F = Gm1m2/r^2
Therefore, since r doubles, the gravitational field strength decreases by 1/4.
9.81/4 = 2.4525 so answer should (hopefully) be B.

Hope this helps!

Reply 89

I've gone totally wrong on a question (same paper as above) but I can't see where my method is different to that on the mark scheme - can anyone point out where I have gone wrong please?

Show that the GPS satellite takes about 40 000s to complete one orbit of earth
mass of earth = 6 x 10^24 kg
radius of earth = 6400m
Height of satellite above Earth's orbit = 20200 m

My method:
F=Gm1m2/r^2
F = mrw^2
rw^2 = Gm/r^2
w = sqrt(Gm/r^3)
w = sqrt ((6.67 x 10^-11 x 6 x 10^24)/(6400+20200)^3)
w = 4.611
T = 2pi/w
T = 2pi/4.611
T = 1.33s

I have obviously gone astray somewhere but I can't see where! Any help would be hugely appreciated.

Reply 90

Original post by candycake

Yes the answer is B :smile:

Thank you so much for solving the question.

Reply 91

Original post by candycake

Um, shouldn't the radius and distance be converted to m from km?

Reply 92

Original post by sabahshahed294

Um, shouldn't the radius and distance be converted to m from km?

Oh yeah - I forgot about that!

Although, even when I convert the units:
sqrt ((6.67 x 10^-11 x 6 x 10^24)/(6.4+20.2)^3) = 146000
2pi/146000 = 4.3 x 10^-5 s

Is there somewhere else I've gone wrong?

Reply 93

Zacken

Original post by candycake

Is there somewhere else I've gone wrong?

You've misread the question. The radius of the earth is 6400 km. And the height is 20200 km. Which you need to convert to SI units, metres. It should have been a red flag because can 6400 metres really be the radius of the earth...?

(edited 7 years ago)

Reply 94

Original post by Zacken

Ah, got it now! Thank you!

Reply 95

Hi. Can someone help out in Q 18a(iii)?
https://a4942901ab27cf2817f7a4f7497d95e45091f796.googledrive.com/host/0B1ZiqBksUHNYTU9SWFFTVjI5T0k/June%202011%20QP%20-%20Unit%205%20Edexcel%20Physics.pdf

Reply 96