# Trouble Determining Moment Arm (aka Lever Arm)

ΣM=Στ

## 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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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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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!

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.

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 ?

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 .

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:
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.