Don't mechanical advantage disagree with the law of nature ?

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Discussion Overview

The discussion centers around the concept of mechanical advantage in physics, particularly in relation to the laws of energy and work. Participants explore how mechanical advantage allows for reduced force when lifting weights using systems like pulleys and ramps, while questioning the implications of energy conservation in these scenarios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about mechanical advantage, questioning how energy is conserved when using a pulley system to lift a weight with less force.
  • Another participant explains that mechanical advantage involves trading force for displacement, maintaining that work (force times displacement) remains constant.
  • A different reply reiterates that the same amount of work is done regardless of the force used, as long as the displacement is adjusted accordingly.
  • One participant suggests that in a pulley system, the ceiling provides the additional force needed, while another humorously notes that the ceiling does not do work.
  • An analogy involving an inclined ramp is presented to illustrate how energy usage remains constant despite changes in force and distance.
  • A later response indicates that the initial confusion has been resolved, with the participant acknowledging a better understanding of the relationship between work, force, and displacement.

Areas of Agreement / Disagreement

While some participants clarify the mechanics of work and energy in relation to mechanical advantage, the initial confusion about the concept indicates that there is not complete agreement or understanding among all participants. The discussion includes both supportive explanations and humorous commentary, suggesting a mix of perspectives.

Contextual Notes

Participants reference the relationship between work, force, and displacement, but there are underlying assumptions about the definitions of energy and work that are not fully explored. The discussion does not resolve all uncertainties regarding the implications of mechanical advantage.

ali171
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Don't "mechanical advantage" disagree with the law of nature ?

hello

I just can't swallow the idea of mechanical advantage.

* if you have a 10 KG mass , you need , say , 100 N to left it up
* if your friend helps you , each one of you will exert only 50 N
* but if you are alone , and you have two pulleys in a distinct
configuration , then - again - you need only 50 N

I just want to ask :
in the third case , who is the "friend" that exerts the
other 50 N ?? from where did that energy came from ??

isn't energy "cannot be created" ??
 
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Your not creating energy from anything, your just trading force for displacement.

Work = Force x Displacement

With a mechanical advantage, like a lever or a pulley, your making the displacement part of the equation bigger and making the force part smaller, yielding the same amount of work.
 
Welcome to PF!

ali171 said:
in the third case , who is the "friend" that exerts the
other 50 N ?? from where did that energy came from ??

isn't energy "cannot be created" ??

Hi ali171! Welcome to PF! :smile:

Work and energy are the same thing.

And work = force x distance.

So the same work is done if you halve the force but double the distance.

With the pulley system, one man has to pull the rope twice as far (2 feet of pull for only 1 foot of raising the weight).

So he has only half the force, but the same amount of work. :smile:
 


ali171 said:
I just want to ask :
in the third case , who is the "friend" that exerts the
other 50 N ??
Depending on how you arranged the pulley, it's the ceiling that exerts the other half of the force.
from where did that energy came from ??
As has been explained, it's you doing all the work. (That ceiling isn't much of a "friend"--it can't move and doesn't do any work.)
 


Doc Al said:
(That ceiling isn't much of a "friend"--it can't move and doesn't do any work.)

That and he's always looking down on you!
 


Think of an incline, such as a ramp. If you have a short ramp that goes 1m left and 1m up, and you are pushing 1kg (assume no friction), you push 1kg up to 1m = 10J of energy used.

Next case, ramp goes up 1m, but left 5m. So again you lift it up to 1m, so it's 10J of energy used. But this time the ramp isn't as steep, so it's easier to push, but you have to push longer.

With pulleys, you'll use 50N instead of 100N, but you'll have to pull twice as much rope to get the weight up.
 


first , thanks for the 'welcome' :)

all the replies were just great . now i understand it...
i should though of the work relation with displacement and force

anyway , thanks again .
 

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