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Mr K's OCR (not OCR MEI) Mechanics 2 Answers May 2014

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I did the same thing for 7iv) as i couldn't prove the distance

I proved it then didn't realise at the time I could do 0.654sinx = r and just assumed it was 0.5 :/

I also messed up the energy question, I knew acceleration wasn't constant but forgot W=p/t so I ended up just doing suvat in desperation... Sad times

0.854*

so what's everyone's general feeling for grade boundaries do a poll?

Reply 23

BunjieJumpa

Why is u<0.384 instead of greater than?

(edited 9 years ago)

Reply 24

arrow900

I know this might sound stupid but did anyone write cot(a)> u for the inequality question, where a represents the angle alpha that was ( to the best of my awful memory) given ?

Reply 25

Mr M

Study Forum Helper

Original post by Zak_Dubai

For 6 ii and iii I stupidly used just m, not 2m and 3m as the masses for my calculations; obviously the method for change in KE and using the impulse to find the mass was correct, but do you think I'll drop all marks, or maybe gain 1 out of 7 for method? Was so disappointed the restitution question was so easy, and then it's the one Q I slip up on..

I expect you will get a couple of method marks.

Reply 26

Mr M

Study Forum Helper

Original post by numlock1

For 6(i) I got the two speeds wrong which means that consequentially I got 6(ii) and 6(iii) wrong too because they needed those speeds.

How many marks would I lose for that? All accuracy marks or is there follow-through (assuming my method for part (ii) and (iii) were correct)?

Just lose marks on 6(i) I expect.

Reply 27

Mr M

Study Forum Helper

Original post by StephenJHolmes

I got 4iii as 0.848, it was most likely my rounding errors so I presume I'd have lost a mark for that question... Also I took the radius as 0.5m on 7iv) but I still worked it out resolving forces, am I likely to get the method marks?

Drop a couple of marks on 7iv at least.

Reply 28

Mr M

Study Forum Helper

Original post by BunjieJumpa

Why is u<0.384 instead of greater than?

Because you want it to slide and not topple.

Reply 29

TeslaCoil

Original post by louise_234

if i got the wrong answer to 1i but used if for 1ii in the correct way would i get the marks for part 2? and is 4ii correct? i got 13.2 but i just found the reaction force. Thanks

They usually allow follow through marks if you've used the correct method; you'd probably only lose one mark for the final answer. And it's definitely 17.2, you had to combine the frictional force and the reaction force.

Reply 30

thanifa

For the question of the bar attatched to the wall by the sting and friction was it supposed to be modelled like a freely hinged rod or only having a reaction in the x direction?

Reply 31

Mr M

Study Forum Helper

Original post by thanifa

For the question of the bar attatched to the wall by the sting and friction was it supposed to be modelled like a freely hinged rod or only having a reaction in the x direction?

It cannot just have a reaction in the x direction as the weight of the rod and the particle exceeds the tension (even if we ignore the fact that the tension is not acting vertically) and you were told the system is in equilibrium.

Reply 32

thanifa

Original post by Mr M

Then would the friction be the vertical componant of the reaction or another force?

Reply 33

Mr M

Study Forum Helper

Original post by thanifa

Then would the friction be the vertical componant of the reaction or another force?

The former.

Reply 34

BankOfPigs

For q4 with the rod, would I get any marks for finding the vertical component by receiving for friction in the last part of the question?

Reply 35

Goods

Original post by Mr M

The former.

I think 7)iv) is 16.5 ( rounded from 16.4526 ) if you carry thorough the exact value from part iii)

please say both would get full credit?

(edited 9 years ago)

Reply 36

Mr M (jr)

Original post by Goods

I think 7)iv) is 16.5 if you carry thorough the exact value from part iii)

please say both would get full credit?

Indeed it is. 16.5 or 16.6 would get full credit.

Reply 37

Big-Daddy

Original post by Mr M

Yes.

Mr M, I accidentally switched the directions of action of the friction and normal reaction forces in the rigid equilibrium question. My follow-through was correct from the point of view of this mistake - I got the right magnitude of force etc. - but ultimately ended up with 1/the correct coefficient of friction, for obvious reasons.

How many marks would I lose?