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# How did you do in M2?! watch

1. Yup 2/7u is correct, which is smaller than 2/3u, therefore no collision
2. i didnt think it matters whether its a lamina or a wire system???
3. (Original post by chenners)
For the centre of mass i got 3cm from AB
So taking moments about midpoint of BC gave
0.5 M = 3.5 k M
So 1 = 7 k

k = 1/7 maybeeee??!!!!
yeah! k= 1/7
4. Yeah same here, I don't think it matter if it was a wire or not it still considered as a lamina.
5. Did anyone manage to get simply a vertical component to part b) of the rod question? I managed to get the x-component to cancel out completely and equal zero! I know that is so obviously wrong because they always ask for a bit of pythagoras' for the magnitude of reaction forces...

Can anyone clarify?!
6. (Original post by jmzcherry)
i didnt think it matters whether its a lamina or a wire system???
yes it does. If it is a uniform wire, then the mass will act at the midpoint of each wire. So you take that into consideration.. if it was a lamina, then it is much easier (I started by assuming that it was a lamina,....then changed my answer :O)
7. The only difference between a wire and lamina system is that you'd let 'm' equal mass per unit length, instead of mass per unit area for a lamina
8. Oh and was there a loss of PE and a loss of KE?
9. But surely the centres of masses of the individual wire conform together to give the centre of mass that would be the same as if it were a lamina?
10. (Original post by Mathemagician)
yes it does. If it is a uniform wire, then the mass will act at the midpoint of each wire. So you take that into consideration.
yup, it was like that.
stupidly enough i made a arithmetic mistake that i realized just as time ended, n got an answer of 2 2/3
i feel so bad about that cuz i KNEW i had it written wrong but cudnt change it
o well..the rest of it was all good..that question was easy as well -- i just dont kno how many marks i'd lose for that!
do u think it will just b the answer marks n not the method marks? cuz i used the wrong value for part b as well getting a wrong k
anyway, im attaching the working!
what does everyone think the boundary will be? o n how many marks was the COM question in total?
Attached Images

11. (Original post by simiddd)
Wot did u people get for Question 6 heres my answers
1)a)35
b)14.9
2a)31/11
b)15/77
3a)c=4
b) -36i + 8j
4a) 1.8m
b)6.75
5a) 2/3
b) proof
6a) -----------
b) ---------
c)--------
7a)117.6J
b) 10N
c) 0.393
d) 5.4
what was question 4. was it that one about the bricks falling. I remember getting 1.8 for somethinng but I cannot remember any 6.75
for some of my answers I rounded and did not write rational numbers. will I lose the accuracy marks. I thought that accuracy and exact number were more important in futher pure maths. i got pretty much the same. any ideas about the grade boundaries?
12. (Original post by Eddie K)
The only difference between a wire and lamina system is that you'd let 'm' equal mass per unit length, instead of mass per unit area for a lamina
then why is the centre of mass different for a wire shaped cone, and a lamina(cone) ?

I am 100% sure that it makes a difference as to whetehr it is a lamina or a wire.
13. If you consider just the 4X4 square, by considering them as wires, 4x2+4x2+4x4= 32
By considering them as a lamina 16x2=32
14. (Original post by jmzcherry)
If you consider just the 4X4 square, by considering them as wires, 4x2+4x2+4x4= 32
By considering them as a lamina 16x2=32
thats exactly what makes the difference...it wasn't a square
try the same thing including the triangle n u'll see it wont work
15. it does make a big difference if you take it as a lamina... as then the CoM for each shape is at a different point.
16. jmzcherry yeah, that's what I did
17. was 6b 778 N?

and no you can't treat it as a lamina...
18. (Original post by jmzcherry)
If you consider just the 4X4 square, by considering them as wires, 4x2+4x2+4x4= 32
By considering them as a lamina 16x2=32
Lamina and wire does matter...
Lamina mass for the square is 4x4 unit²
The mass of the wire was 4x3 unit²
this is beacuse there were only 3sides of the square!
19. What sort of raw mark do you usually need for an A?
20. for the first question, part (b) - the new max speed the car can travel up the slope - did anyone get 14∟7∟12 (thats supposed to be 14 and 7/12)?

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