Physics question... fulcrum/moments

Psy-Blade

Psy-Blade

Psy-Blade

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Joined
11 Jun 2009
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443
Hey guys, a quick one that's been bugging me.

If you place a wooden board supported by two scales at either end (let's assume 1m length). You then put a weight closer to one of the scales, would both scales go up equally?

I'd have said no. But I don't know why.

Just a quick edit to say, there's no fulcrum in this case, shouldn't have put it in the title I think >.<
 
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I would have thought that when you place weight closer to one of the scales the weight would register would go up more on that one.

Similar to standing on the centre of a see-saw and moving your centre of mavity to have it move up and down at either end.
 
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Swordfish said:
I would have thought that when you place weight closer to one of the scales the weight would register would go up more on that one.

Similar to standing on he centre of a see-saw and moving your centre of mavity to have it move up and down at either end.
Click to expand...

It's not really like a seesaw though as there's no fulcrum in the centre... That's that I thought at first myself.
 
The weight is shared based on the relative distance between supports.

if you take 'a' as the distance from Ra to the weight and 'b' as the distance from Rb to the weight you get the following relationship

Ra=bW/L and Rb=aW/L

Hopefully this helps :)

KaHn

/edit :- thats assuming simply supported before anyone tries to get technical :)
 
Psy-Blade said:
It's not really like a seesaw though as there's no fulcrum in the centre... That's that I thought at first myself.
Click to expand...

That's irrelevant, the centre of mavity would be asymmetrical favouring the scale to which the weight os closer. That scale would bear a greater proportion of the weight and would read higher than the scale farther away.
 
Great, thanks guys, *puts mind at ease.

And that's a handy equation to remember KaHn. Just need to remember the equation for support Ra relies on the distance Rb is from the weight, not Ra... :D
 
I think this kind of problem depends greatly on factors such as the distance between the scales and the mass of the weight. For example if the scales are very close (next to each other) you would expect moving the weight from one end of the board to the other to make very little difference in the display on the scales (although there would still be a difference) however if they are a mile apart, then putting the weight right next to one set of scales, you would expect it to register the full mass of the object and the other set of scales to not register anything (pretty much).

Been a LONG time since I've done any physics though but I think it's common sense.
 
Took me ages to get moments squared away in my head. For some reason it was one of the more difficult things for me. Even though when the penny finally dropped it was one of the easiest! Could be because I was ALWAYS hungover on my first class of the day at college!
 
You've got two fulcrums of variable height. (Does this even matter, I suppose the system reaches equilibrium regardless?)

I give up.
 
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They better not go up together or all that corner weighting of competition cars suspension has been a waste of time. :p


Answer. NO of course the scales wouldn't register a similar increase. You have favoured more weight to one than the other.
 
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