Trouble Determining Moment Arm (aka Lever Arm)

  • #1
84
1

Homework Statement


1.png


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)
1.png


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

Answers and Replies

  • #2
2
0
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!
 

Attachments

  • #3
Nidum
Science Advisor
Gold Member
2,990
848
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 ?.
 
Last edited:
  • #4
84
1
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!
IMG_0270.jpg
 
  • #5
84
1
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.
 
  • #6
84
1
This is messy, sorry, but I'm at this point in trying to figure the question out!
IMG_0271.jpg
 
Last edited:
  • #7
Nidum
Science Advisor
Gold Member
2,990
848
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 ?
 
  • #8
84
1
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
 
  • #9
Nidum
Science Advisor
Gold Member
2,990
848
Yes - that angle and angle A1 are the same . Quick bit of trig now will give you length of blue line ?
 
  • #10
Nidum
Science Advisor
Gold Member
2,990
848
Hint : Work out the base length of the triangle below point P .
 
  • #11
84
1
Hint : Work out the base length of the triangle below point P .
Tried it this way, still not getting correct answer
IMG_0272.jpg
 
  • #12
Nidum
Science Advisor
Gold Member
2,990
848
1-png.96777.png


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:
  • #13
84
1
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.
IMG_0275.jpg
 

Related Threads on Trouble Determining Moment Arm (aka Lever Arm)

  • Last Post
Replies
3
Views
8K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
10
Views
2K
  • Last Post
Replies
5
Views
2K
Replies
1
Views
8K
  • Last Post
Replies
3
Views
20K
  • Last Post
Replies
11
Views
960
Replies
3
Views
9K
  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
1
Views
2K
Top