M3 question help

Original post by Nathanbh321

Can anyone help me see where I'm going wrong in this question, it's January 2014 IAL Q5. It's gotten to the point where I've convinced myself I'm right even though I know I'm wrong 😓

where is the question?

Reply 3

Original post by Nathanbh321

Can anyone help me see where I'm going wrong in this question, it's January 2014 IAL Q5. It's gotten to the point where I've convinced myself I'm right even though I know I'm wrong 😓

M(O), the weight should be just mg I think, not mg sin theta.

Reply 4

Q5 here: https://8a40d6c38bafca75cc407741c0f3e1889c8e66b2.googledrive.com/host/0B1ZiqBksUHNYWENTbktxdlk5dEU/January%202014%20(IAL)%20QP%20-%20M3%20Edexcel.pdf

Original post by TeeEm

where is the question?

Reply 5

OP

Original post by Zacken

M(O), the weight should be just mg I think, not mg sin theta.

But then it wouldn't be perpendicular to the line of action of mg, would it? I'm so confused. You could take the sin(a) of the whole thing but that yields the same result 😞

Reply 6

Original post by Nathanbh321

But then it wouldn't be perpendicular to the line of action of mg, would it? I'm so confused. You could take the sin(a) of the whole thing but that yields the same result 😞

I haven't checked properly - but it's either the fact that your resolving upwards on the first line should be mgsin theta or your weight in your moments equation should be mg. It's one of those two problems, I think

Reply 7

Original post by Nathanbh321

But then it wouldn't be perpendicular to the line of action of mg, would it? I'm so confused. You could take the sin(a) of the whole thing but that yields the same result 😞

Original post by Zacken

I haven't checked properly - but it's either the fact that your resolving upwards on the first line should be mgsin theta or your weight in your moments equation should be mg. It's one of those two problems, I think

If you do not get joy give me a shout and I will post my solution to the paper

Reply 8

OP

Original post by Zacken

I haven't checked properly - but it's either the fact that your resolving upwards on the first line should be mgsin theta or your weight in your moments equation should be mg. It's one of those two problems, I think

The mg wouldn't give the correct solution either, so it couldn't be that. In the mark-scheme it gives (9/4rsin(theta) - rcos(theta))*mg as the moment, as if the sin(theta) was only applied to the 9/4r. Is that possible?

Reply 9

OP

Original post by TeeEm

If you do not get joy give me a shout and I will post my solution to the paper

Yeah that would be great, thanks.

Reply 10

Original post by Nathanbh321

Yeah that would be great, thanks.

let Zacken help you first ...

Reply 11

Original post by Nathanbh321

The mg wouldn't give the correct solution either, so it couldn't be that. In the mark-scheme it gives (9/4rsin(theta) - rcos(theta))*mg as the moment, as if the sin(theta) was only applied to the 9/4r. Is that possible?

Right - that's exactly what you need to do. 9/4 r sin theta is the perpendicular distance, which is what you need to use for the moment equation.

Reply 12

OP

Original post by Zacken

Right - that's exactly what you need to do. 9/4 r sin theta is the perpendicular distance, which is what you need to use for the moment equation.

So I tried this and got the same result, cos(theta), 9/4 rsin(theta) is longer than the perpendicular distance so as shown you need to take away rcos(theta)sin(theta)

Reply 13

Original post by TeeEm

let Zacken help you first ...

Go ahead.

Reply 14

Original post by Zacken

Go ahead.

Original post by Nathanbh321

Yeah that would be great, thanks.

Reply 15

OP

Original post by TeeEm

Thanks, I felt like something vital was missing. Didn't think of the other component of the weight 😜. Idk what's wrong with my method but I can work with this.

Reply 16

Original post by Nathanbh321

Thanks, I felt like something vital was missing. Didn't think of the other component of the weight 😜. Idk what's wrong with my method but I can work with this.

It can be done without resolving weights, but then you need to resolve distances
you are looking at my style which might not suit everybody ...
but I think it is very clear since ...

Spoiler

Reply 17

OP

Original post by TeeEm

It can be done without resolving weights, but then you need to resolve distances
you are looking at my style which might not suit everybody ...
but I think it is very clear since ...

Spoiler

This is either a reference to a song I can't quite remember, or you're being very flirtatious

Reply 18

Original post by Nathanbh321

This is either a reference to a song I can't quite remember, or you're being very flirtatious

you are probably too young
https://www.youtube.com/watch?v=PNA7DcVppEs

Reply 19

OP