OCR Mechanics 1 25th Jan

Mr M, on Q4 I found the T=3.36N and a=1.4 for the pulley. I then used them to find R=0.28 and F=0.49 (apparently wrong). On dividing to find mu, I got 1.75. Was there a quick way to find R and F without having to find T and a?

I don't know what you did there. The system is in equilbrium so clearly the tension must be the same as the weight of Q (2.94 N).

http://en.wikibooks.org/wiki/A-level_Mathematics/M1/Equilibrium_of_a_Particle Under friction, it has a limit on mu. Dammit.

I think the website is wrong, or worded badly or I didn't read it properly. Either way, I read the article the night before the exam and thought that mu had max value of 1 - spent a good 15mins thinking that I had done something wrong in that q4 when instead I could have have checked my q9...

I don't know what you did there. The system is in equilbrium so clearly the tension must be the same as the weight of Q (2.94 N).

read post above....sorry i thought id quote you to grab your attention haha.....thanks again

Ok very briefly.

Look at Q. It has weight 2.94 N and is in equibrium so the tension in the string is also 2.94 N.

The weight of P is 3.92 N.

Perpendicular to plane:

R + 2.94 cos 60 = 3.92 cos 60

R = 0.49 Newtons

Parallel to plane:

3.92 cos 30 = 2.94 cos 30 + Friction acting up slope

Fric = 0.49 sqrt 3 Newtons

mu = Fric / R = sqrt 3

(Or you can take a short cut and just type tan 60 into your calculator!)

OCR (not OCR MEI) answers January 2010


All non-exact answer given to 3 s.f.


1. (i) Speed of P at ground = 18.9 m/s (2 marks)

(ii) Height = 17.3 m (2 marks)

(iii) Speed of P when 5 m above ground = 16.1 m/s (2 marks)


2. (i) Magnitude of resultant = 27.1 N (5 marks)

(ii) Angle = 37.4 degrees (3 marks)


3. (i) m = 0.096 kg (4 marks)

(ii) Change in momentum = 1.2 Ns (2 marks)

(iii) Common speed = 3.25 m/s (3 marks)


4. (i) (a) R perpendicular = 0.49 N

R parallel = 0.849 N (4 marks)

(b) Coefficient of friction = 1.73 (2 marks)

(ii) T = 4.36 N a = 1.09 m/s² (4 marks)


5. (i) t = 12.5 seconds (3 marks)

(ii) t = 26.7 s (5 marks)

(iii) Sketch graph (3 marks)


6. (i) a = 0.012t - 0.18 (2 marks)

(ii) Show k = 2 (5 marks)

(iii) x = 0.002t³ - 0.09t² + 2t

x = 30.0 m (5 marks)


7. (i) T = 1960 N (5 marks)

(ii) (a) Show 3.15 (2 marks)

(b) t = 9.50 seconds (9 marks)


Edit: question 7 fixed.

Mr M,, Thanks as always for the answers!

I thought this paper was slightly above standard difficulty, but it was fair enough.

I was very annoyed about question 4) I have been led to believe that the co-efficient of friction never exceeds 1, and i had never seen a question where mu > 1. Therefore on reaching my answer of 1.73, i crossed it out, and made up a load of rubbish to get what i thought was a reasonable answer of .43ish.

Do you not think that it was very unfair to throw in a question with a mu > 1? I do

Most other question were fine!

2 quick queries:
1) I really cant remember the change in momentum question. Any chance you could show your working for this one. Obviously it's simple enough, but i can't remember the values used and I don't regonise the answer.

2) For 4ii Why was tension 4.36N. Now that q = 0.5kg, does T not equal 0.5g N = 4.9N?

Thanks for all the great help
x

oh god I didnt think you needed to take Q into account........would i possibly still get method marks for resolving P only...and also would i get a mark on the next bit for stating coeff of friction = F/R but because I had the wrong values I got the wrong answer.....orr will they be nice and carry the marks forward so whatever u got for F and R then if u plugged it in it works? thanks bit confusing sry

What marks did people get in this. Everyone was complaining including me i think about the wording and the coefficient of friction, but somehow i managed 86!
Post what you expected and what you got. I think the mark scheme must of been quite low!

I got 99/100 UMS :O

Congrats, what did you truthfully expect?

Honestly, I put a lot of work into mechanics, and found the paper ok, so I did expect a high A.. just not that high :P!

great stuff, its so weird to be told at my school noone has ever surpassed 91 in c4, then i did well for 86. Then you come on tsr and feel like a right failure! So many clever people on here haha.
Nice isnt it when the hard work pays off! I only need three Cs now for AAB to manchester so im happy! Well done!

Mate, I wish all my grades were like my mechanics one :')
I'd be chuffed with 86 in C4!
My other maths grades were: C1: 81, C2: 74, C3: 84, S1: 97 M1: 99

So as you can see, they're not all as high as M1, but I'm still v.pleased :P

Well, I did get a 100 in one of my exams though M1 was my tied worst

Which was what 99? :P