How to measure a force on a pivot (remedial)

[kindejj]

Hi all,

I apologize now at how elementary this is, but I have been all over google and I cannot get a straight answer. My question seems to me be to be quite easy. Maybe I am over simplifying it. Anyways, any help is much appreciated. Btw, it's been 15 years since my last physics class, so be patient please!

If I have 3 levers attached to the same pivot, one at 45 degrees, one at 0 degrees at one at -45 degrees, each lever being 1 meter long, and a 100 gram mass at the end of each. How do I go about measuring the force that each lever is applying to the pivot?

Thanks in advance. See attachment for example.

 

[tiny-tim]

Welcome to PF!

hi kindejj! Welcome to PF!

erm … they aren't levers!

a lever has a fulcrum (a pivot will do), and two other points: one point for "inputting" a force, and one point for "outputting" a force

yours don't

what engineering situation were you envisaging? ​

 

[sophiecentaur]

Hi. I am assuming that the lines in your diagram all represent rigid rods, rigidly joined together (A rigid body). So far, the system is not in equilibrium - because the applied forces are all 'downwards' and on the same side. It's anyones guess what will happen because you haven't specified enough about the problem (except to say that the frame will rotate anticlockwise if it has some mass).
Wiki can't answer such a loose question and neither can I. Tighten it up a bit and we're in with a chance.

 

[kindejj]

I don't have an application, as of yet. I am just trying to further my (obviously) basic understanding. Let me ask the question in a different way. Maybe this will help. Look at this new drawing. Is it in equilibrium? If not, why?

Assume that the red and blue lines are solid rods of equal length attached to a solid axle (the center black dot), that rotates freely, leave friction out of it for now. Also assume the yellow boxes are the same mass.

How are the forces calculated? I assume the red rod loses some of the rotational force applied to the "axle" (IDK what to call it) due to the angle. I mean, if it were perfectly verticle on the axle it would not rotate, right?

I guess I am asking even if the masses are the same, does the angle come into play, throwing the system out of balance, and if so, how does one go about calculating that?

Thanks for the earlier replies!

 

[tiny-tim]

i see … so you have a horizontal axle with spokes with weights on the end, and you want to know whether the axle will rotate

first, in a lever the force is usually perpendicular to the lever, but the forces here (the weights) are all in the same direction, vertical …

in this case the axle will rotate if the centre of mass is not directly above the axle …

to find the centre of mass, multiply each mass by the horizontal distance from the pivot …

that's where the angle comes into play ​

 

[sophiecentaur]

Why not read up about the Principle of Moments in Wiki or a textbook? That will tell you all you need and you can see lots of examples all over the web.

 

[kindejj]

"Why not read up about the Principle of Moments in Wiki or a textbook?"

Because I don't know what I am looking for. Which is why I am asking here. Sorry for disturbing you!

 

[sophiecentaur]

You do now, 'cos I've told you what it's called.

Also 'levers' would be a good topic.