You are Here: Home >< Maths

# m1 help Watch

1. In the Mark scheme they have took moments about C but I don't know how they get 3gcos30? how is that the force x perpendicular distance from c? I c
e
cant see it..
edit: would it not be 3g(cos30)(2)?
2. The 3g cos(30) is the force 3g * 1 cos(30), because the perpendicular distance is the same as the arrow marked "R" (even though they're unrelated in the diagram, it's a useful marker for me to use on the diagram.)
3. (Original post by Smaug123)
The 3g cos(30) is the force 3g * 1 cos(30), because the perpendicular distance is the same as the arrow marked "R" (even though they're unrelated in the diagram, it's a useful marker for me to use on the diagram.)
but is R not 2m away from c?
4. (Original post by upthegunners)
but is R not 2m away from c?
Yes, it is - I was only using "R" as a conveniently drawn arrow on the diagram, not as representative of the force R. g is cos30 away from c, because the "perpendicular distance" from g's action to c is cos30; imagine drawing a dotted line along what is currently marked 3g, all the way off the top of the page. At one point, the dotted line draws level with C; at that point, the distance between the dotted line and C is cos30, and that's the perpendicular distance. (because the distance between BC and the dotted line is constant, and at the point B that distance is cos30.)
2m might be called the "parallel distance".

EDITED TO ADD: See ghostwalker's diagram, which is exactly what I was trying to describe; a picture speaks a thousand words!

EDITED AGAIN TO ADD: Said diagram seems to have disappeared - maybe I'm going mad…
5. See, in green.

Reposted.
Attached Images

6. (Original post by ghostwalker)
See, in green.

Reposted.
thanks for the diagram but I dont dont get why it is? can we not just do 3g and not have to times it by anything since (as you have showing in your diagram) 3g is already perpendicular to c?
7. (Original post by upthegunners)
thanks for the diagram but I dont dont get why it is? can we not just do 3g and not have to times it by anything since (as you have showing in your diagram) 3g is already perpendicular to c?
As Smaug123 explained. The moment of a force is the product of the force times the perpendicular distance of it's line of action from the fulcrum.

In this case the perpendicular distance is represented by the line in green, which is
8. (Original post by ghostwalker)
See, in green.

Reposted.
9. (Original post by Smaug123)
EDITED AGAIN TO ADD: Said diagram seems to have disappeared - maybe I'm going mad…
I deleted and reposted it. Keep calm.
10. (Original post by ghostwalker)
As Smaug123 explained. The moment of a force is the product of the force times the perpendicular distance of it's line of action from the fulcrum.

In this case the perpendicular distance is represented by the line in green, which is
so if 3g was at point A in the diagram it would be 2 x cos30?
11. (Original post by upthegunners)
Yes, the cos 30 comes from where the arrow is.

Or you could use sin 60, as more obvious.
12. (Original post by upthegunners)
so if 3g was at point A in the diagram it would be 2 x cos30?
That's correct.
13. (Original post by ghostwalker)
That's correct.
you're a brilliant help

thanks very much
14. (Original post by ghostwalker)
I deleted and reposted it. Keep calm.
Phew thank goodness for that!

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 9, 2013
Today on TSR

### How to stand out in an Oxbridge interview

What makes you memorable?

### A pregnant woman visits me every night

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE