Finding smallest forces required to produce a given moment

[Greg Needel]

i was going to go to bed but now you have be thinking about this...give me a few mins and i should have something forward. My initial thoughts are about the fact that Fa and Fd are equal and act in the same direction so using Sum of the moments= 0 you can get your required force....let me play with this a bit.


edit: sorry dude my brain isn't working this second but i will look at it in the morning for ya. I hope it isn't due at 8 am

[sanddrag]

Originally Posted by Greg Needel

edit: sorry dude my brain isn't working this second but i will look at it in the morning for ya. I hope it isn't due at 8 am

Don't worry. Thanks for coming back to tell me so instead of just disappearing. Actually, it is due in the morning but I don't care about getting points for it. I only want to understand.

Now, can you honestly sleep at night with this problem lingering on your mind? I know I can't. Which is why hopefully someone will come along soon with an explanation.

[Leo M]

The Given Moment at P is Mp = 10 lb-ft.

a) Corners A and D are 10” apart = 0.833 ft.

The equivalent moment is Force X Distance = Fad X 0.833 ft = 10 lb-ft.

So, smallest F = 10 lb-ft/0.833 ft. = 12 lb, applied perpendicularly to side AD.

b) Corners B and C are 11.66 inches apart = 0.97 ft.

The equivalent moment is Fbc X 0.97 ft = 10 lb-ft

So, smallest Fbc = 10/0.97 = 10.3 lb., applied perpendicularly to side BC.

c) The greatest distance between two points on the block is from A to C, 18.867 inches = 1.572 ft.

The equivalent moment is Fac X 1.572 ft = 10 lb-ft

So smallest Fac = 10/1.572 = 6.36 lb, applied perpendicularly to the line AC.

Hope this helps and is in time for you - dashed it off quickly this AM - have to run.

Good luck.

[Leo M]

I have a diagram in the attached Word file that may help some. Just remember that a moment is a moment wherever on the object it is applied, as long as the equivalent forces are coplanar and the plane is the same as for the original moment applied to the object.

[sanddrag]

You are magnificent! Thanks so much. I totally understand now. I knew it was something simple I was missing. For parts (b) and (c) I wasn't applying the forces perpendicular to the longest distance between the points. I knew this wasn't a hard problem but I couldn't understand why I wasn't getting it, but now I understand it no problem and I took a look at the rest of my problems and I don't think they will be any problem for me. (I've already done 3D moment and force equilibrium problems successfully, but this simple 2d one got me).Anyway, thanks again for the help! It is greatly appreciated.

[Leo M]

Glad I could help. Let me know when you get to trusses - those are tricky. And, it's really fun when you get to virtual work and strength of materials - the statics stuff gets boring real fast!