Trouble Determining Moment Arm (aka Lever Arm)

[x2017]

Homework Statement

Homework Equations

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).

 

[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.

Hope this answers your question!

 

[Nidum]

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 ?.

 

[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.

Hope this answers your question!

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!

 

[Nidum]

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]

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

 

[Nidum]

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]

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 .

 

[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.

Hope this answers your question!

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.
Thanks for your help.