Most CAN'T solve this A Level Physics Question

OCR 2020 Modelling Physics Paper 1, Question 10. This is a question, which the examiner report highlighted as one which most candidates had trouble with. Let's solve it!
Please note that this is not an official solution. Have a look at the paper here:

[zhelyo_physics]

Watch closely and you might even spot Darth Vader somewhere in this video

[fariajahangir]

this came up in my aqa exam as multiple choice today and i saw this video yesterday, thank you so much!

[udayreddy]

so sir the reason we used e = 1/2kx^2 is because k would be constant, so we can use the proportionality idea whereas for e = 1/2fx, the f and x both change so we can't say they are proportional?

[Nip]

Glad to see my rusty mental physics still holds up

[youssefts]

Exactly, why C isn't the correct answer?

[KazACWizard]

the first equation you used was wrong because the fact that the force changes. this means that mass AND extension change to do that and any deltaM changes deltaX so mass is proportional to extension. mass squared is proportional to extension squared plussing that in we get E is directly proportional to X^2?

[mohamedwaziry]

We need the antimatter formules and equations and researches .❤❤❤👍

[tusharjawane]

Should we evaluate x from the equilibrium position to position of max extension as in spring block system the velocity of the block is zero at max extension meaning it is momentarily at rest?
And of course I tried it and got the same answer

[shimiwinegarten]

I watched this this morning, came up in the exam, if m is replaced by 2m, whats e

[nidhi]

Why?😭 I don't get why it's not correct

[two]

Ngl this question wasn't hard in the slightest. I've definetly done many harder questions in A level physics

[fatimabatool]

Can I just use your videos and past papers for revision or should I also use textbook? I am aiming for an A* and I am doing OCR btw thank you soo much for all the video they help alot.