Physics Help Forum Questions on confusing lever mechanics in the real world (not mathematical)
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 Jun 28th 2018, 07:21 PM #1 Junior Member   Join Date: Jun 2018 Posts: 4 Questions on confusing lever mechanics in the real world (not mathematical) This is based on a confusing question my brother asked out of curiosity. I know how levers work, i.e., I know they trade distance for power and vice versa. What we want to know is why that actually works in the real world, because thinking about it, it doesn't seem to click. My brother has a theory that it's based on time travel and the speed of light with the distance moved and all, but I'm skeptical. How a gear works makes sense, but this somehow doesn't.
Jun 29th 2018, 03:38 AM   #2
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 Originally Posted by HeliosV This is based on a confusing question my brother asked out of curiosity. I know how levers work, i.e., I know they trade distance for power and vice versa. What we want to know is why that actually works in the real world, because thinking about it, it doesn't seem to click. My brother has a theory that it's based on time travel and the speed of light with the distance moved and all, but I'm skeptical. How a gear works makes sense, but this somehow doesn't.
It seems like your brother is not taking you seriously. He's probably just winding you up.

 Jun 29th 2018, 04:15 AM #3 Senior Member   Join Date: Aug 2010 Posts: 403 Levers use the fact that energy is conserved, not force. If you move an object using a lever such that the distance from the fulcrum to the point at which is applied is twice the distance from the fulcrum to the object then, since energy is "force times distance", the end where you are applying the force will move twice as far as the end at the object. The energy is the same at both ends but the force at the "working end" is twice the force you apply.
Jun 29th 2018, 06:19 AM   #4
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 Originally Posted by HeliosV ...my brother has a theory that it's based on time travel and the speed of light with the distance moved and all, but I'm skeptical. How a gear works makes sense, but this somehow doesn't.
Like benit said, it sounds like your brother is winding you up. To get back at him, tell him time travel is science fiction for thickos and kids.

That's because "the time" is nothing more than a cumulative measure of some kind of regular cyclical motion, such as the motion of cogs and gears inside a clock. And because you can't travel through a measure of motion.

Yes, people talk about travelling forward in time, but that's just a figure of speech. So is "time flows". It doesn't. A clock isn't some cosmic gas meter gizmo with time flowing through it. If he starts iffing and butting and ducking and diving, tell him to read this: the nature of time. That should shut him up.

 Jun 29th 2018, 08:04 AM #5 Junior Member   Join Date: Jun 2018 Posts: 4 My brother is my brother, but it does make sense in some kind of way. Ok new example: if 2 weights weigh 50 pounds each, and they are placed on opposite sides of a lever and fulcrum, why does one weight lift up when the forces exerted are the same? Last edited by HeliosV; Jun 29th 2018 at 10:51 AM. Reason: new info so people wont be like "wELL..."
Jun 29th 2018, 04:58 PM   #6
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 Originally Posted by HeliosV My brother is my brother, but it does make sense in some kind of way. Ok new example: if 2 weights weigh 50 pounds each, and they are placed on opposite sides of a lever and fulcrum, why does one weight lift up when the forces exerted are the same?
If the weights are placed at the same distance from the fulcrum, the two weights balance- one does NOT go up while the other goes down. If one is placed further from the fulcrum, it exerts a greater torque (weight times distance from the fulcrum) and will go down.

Jul 2nd 2018, 02:25 AM   #7
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 Originally Posted by HeliosV My brother is my brother, but it does make sense in some kind of way. Ok new example: if 2 weights weigh 50 pounds each, and they are placed on opposite sides of a lever and fulcrum, why does one weight lift up when the forces exerted are the same?
This is a see-saw.

If the equal weights are placed at equal distances away from the pivot point, then the torques each one applies are equal and opposite, so no rotational acceleration occurs.

If the distances from the pivot point are different, then because torque is

$\displaystyle \tau = r \times F$

and even though F is the same, r is not, so the torques applied by each weight differ. Therefore, there will be a net torque equal to the difference in the individual torques, the see-saw will undergo rotational acceleration and it will tip over.

You can do this experiment for yourself. Make a see-saw using a long, wooden, ruler and a big blob of blue-tac. Then, get two different weights and figure out the two different distances that both can be placed so that the torques are the same. You'll find that the see-saw either doesn't tip over or it tips over slowly (because of inaccuracy). Another way to do it is to go to a see-saw in a park and find the bit of the see-saw you need to sit to balance it (unless you weight the same as your brother!)

Last edited by benit13; Jul 2nd 2018 at 02:30 AM.

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