# Trouble Determining Moment Arm (aka Lever Arm)

#### x2017

1. Homework Statement

2. Homework Equations
ΣM=Στ

3. The Attempt at a Solution
I know how to get the net torque about point P, but I am having difficulty determining where the moment arms for each force should be. I did 9 simpler questions before this no problem, but this diagram is a bit more complicated and I'm having trouble with it. The last three questions I have to do are all exactly the same (this one) with different numbers, so some help would be greatly appreciated!

I know the moment arm is the perpendicular distance from the line of action of the force to the point of rotation.

I am confused as how to find the length of the moment arms in this particular diagram. I think they should look like this (orange and blue lines)

But I am lost as to how to get their lengths (if where I have them is even correct).

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#### ShraZam

Hi,

Perhaps you could try to divide the forces F2 and F1 in the x and y directions.

Such that F2x = F2cos(A2), F2y = F2sin(A2), F1x = F1cos(A1), and F1y = F1sin(A1).

Hence, Your free body diagram would look a little like the attached picture.

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#### Nidum

Gold Member
Look at the point where your blue line crosses the horizontal line at the bottom of the rectangle .

What do you know about the two acute angles there ?

What do you know about the sum of the angles in right angle triangles ?.

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#### x2017

Hi,

Perhaps you could try to divide the forces F2 and F1 in the x and y directions.

Such that F2x = F2cos(A2), F2y = F2sin(A2), F1x = F1cos(A1), and F1y = F1sin(A1).

Hence, Your free body diagram would look a little like the attached picture.

I've tried this a few time and I haven't been able to get the correct answer :(
Do you spot a mistake I could have made?
If you can't read my writing I can type it out!

#### x2017

Look at the point where your blue line crosses the horizontal line at the bottom of the rectangle .

What do you know about the two acute angles there ?
I'm not sure, are they supposed to be the same?

What do you know about the sum of the angles in right angle triangles ?.
They add up to 180 degrees.

#### x2017

This is messy, sorry, but I'm at this point in trying to figure the question out!

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#### Nidum

Gold Member
Yes - they are the same . So what is the angle between your blue line and the left vertical line of the rectangle at point P ?

#### x2017

Yes - they are the same . So what is the angle between your blue line and the left vertical line of the rectangle at point P ?
38.3 degrees

#### Nidum

Gold Member
Yes - that angle and angle A1 are the same . Quick bit of trig now will give you length of blue line ?

#### Nidum

Gold Member
Hint : Work out the base length of the triangle below point P .

#### x2017

Hint : Work out the base length of the triangle below point P .
Tried it this way, still not getting correct answer

#### Nidum

Gold Member

This was your best diagram . Put in the actual dimensions and angles as given and add those that you have worked out so far .

Label all points where lines meet or cross . Simple a , b , c --- will suffice .

Last edited:

#### x2017

Hi,

Perhaps you could try to divide the forces F2 and F1 in the x and y directions.

Such that F2x = F2cos(A2), F2y = F2sin(A2), F1x = F1cos(A1), and F1y = F1sin(A1).

Hence, Your free body diagram would look a little like the attached picture.

This was your best diagram . Put in the actual dimensions and angles as given and add those that you have worked out so far .

Label all points where lines meet or cross . Simple a , b , c --- will suffice .
I eventually solved it using the first method I tried (breaking into x and y components.

"Trouble Determining Moment Arm (aka Lever Arm)"

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