How was Edexcel M5?

Daft Wader

Here's what I did, it may be right may be wrong:

xi+yj+zk x R = G

| i j k |
| x y z | = 7i-3j+8k
|-1 3 2|

equating components of i, j and k:
2y-3z = 7
-(2x+z) = -3
3x+y = 8

letting x=0 : y=8, z=3

line of action is therefore
r = 8j+3k +k(-i+3j+2k)

Humm, I used the same method, but I made it equal to the SUM of the couple caused by teh two forces AND the one given in the question, so it wasn't = 7i-3j+8k... hummm

My answers where:

1. 39 J

2. r = 3je^-t + (2i - 4j)e^-2t

3. a) -i + 3j + 2k
b) r = i + 9j + 4k + λ(-i + 3j + 2k) (probs wrong, though)

4. a) Proved it, though it sounded VERY VERY easy for seven marks - just quoted things from formula book and too me four lines. Think I did smthing wrong
b) 11ω/19

5. a) 7mg/4
b) 2 Pi * √( 81 / 3g√2) - that's very obviously wrong, though !

6. a) Proved
b) Tried, but couldn't do it

7. a) Used integration, but got the wrong answer
b) angel = 18.38 degrees (wrong, I think)
c) Ran out of time

Reply 21

herman368

OP

danguetta

Humm, I used the same method, but I made it equal to the SUM of the couple caused by teh two forces AND the one given in the question, so it wasn't = 7i-3j+8k... hummm

My answers where:

1. 39 J

2. r = 3je^-t + (2i - 4j)e^-2t

3. a) -i + 3j + 2k
b) r = i + 9j + 4k + λ(-i + 3j + 2k) (probs wrong, though)

4. a) Proved it, though it sounded VERY VERY easy for seven marks - just quoted things from formula book and too me four lines. Think I did smthing wrong
b) 11ω/19

5. a) 7mg/4
b) 2 Pi * √( 81 / 3g√2) - that's very obviously wrong, though !

6. a) Proved
b) Tried, but couldn't do it

7. a) Used integration, but got the wrong answer
b) angel = 18.38 degrees (wrong, I think)
c) Ran out of time

I agree with 1,2,3a,5a.... ur 5b has a square root in a root so its similar to mine.... i disagree with ur 3b 4b and 7a

Reply 22

tktaylor6

i think the angle was 60 degree's also the lien of action i got the value like you guys did that was like 4j -k or something liek that but i left it as that i didnt times that by lamda and add the force, isnt the line of force the direction its acting...? apart from that i did everything

Reply 23

yazan_l

5

danguetta

4. a) Proved it, though it sounded VERY VERY easy for seven marks - just quoted things from formula book and too me four lines. Think I did smthing wrong
b) 11ω/19

sorry but part b is wrong:
it's 77w/109

Reply 24

Daft Wader

yazan_l

by the way: for the pendulum period question, which should be used:
T = 2π √[I/(mgh)] OR T = 2π/w?

The only time I've seen it make a difference is when they specify which to use on the paper, eg. "by considering the equation of motion..." They didn't specify so it shouldn't matter. I did T = 2π/w, with w²=(8g/3h) or something like that, and got the same answer as everyone else at my centre.

Reply 25

yazan_l

5

Daft Wader

The only time I've seen it make a difference is when they specify which to use on the paper, eg. "by considering the equation of motion..." They didn't specify so it shouldn't matter. I did T = 2π/w, with w²=(8g/3h) or something like that, and got the same answer as everyone else at my centre.

well actually i think that i had a calculating mistake or something ( i used T = 2pi rt[I/mgh]), so i didnt have the same answer when i used the T = 2π/w method. :biggrin: