Work-Energy Theorem: Is Lifting a Rigid Body Really 0 Work?

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Discussion Overview

The discussion revolves around the application of the Work-Energy Theorem in the context of lifting a rigid body vertically against gravity. Participants explore the implications of work done in relation to kinetic energy changes and gravitational forces, questioning the apparent contradiction between theoretical assertions and practical experiences.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant asserts that lifting a rigid body slowly results in zero net work according to the Work-Energy Theorem, as the change in kinetic energy is zero.
  • Another participant challenges this by emphasizing that work is done against gravity, suggesting that positive work is performed even if the net work is zero.
  • A further contribution clarifies that while the participant has done positive work, gravity has done negative work, leading to a net work of zero.
  • One participant questions whether the work done by the individual is equivalent to the net work, prompting further exploration of the definitions involved.
  • Another participant highlights the necessity of specifying the system in which work is being done, suggesting that the work should be considered in the context of the object-Earth system rather than just the object alone.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of work done in lifting a rigid body, with no consensus reached on the relationship between work, kinetic energy, and gravitational forces.

Contextual Notes

Participants note the importance of defining the system in which work is performed and the roles of various forces acting on the object, indicating potential limitations in the initial framing of the problem.

PeterPeter
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I read on the Internet that the work done by a (rigid) body = the change in Kinetic energy.

What if I lift a rigid body slowly and vertically by 1 meter above the Earth's surface so that the initial velocity = final velocity =0?

According to the Work Energy theorem as stated on many sites on the Internet (you can google these for yourself) the net work done = 0 because the change in KE =0.

Yet I have done work against gravity. Clearly something is wrong! It's all very confusing!
 
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PeterPeter said:
Yet I have done work against gravity.
You have done positive work, gravity has done negative work, the net work is zero. Alternatively, you have done positive work that went into gravitational potential energy.
 
PeterPeter said:
the net work done = 0 because the change in KE =0.

Yet I have done work against gravity.
Is the work you did equal to the net work?
 
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It is helpful (necessary) to specify the system on which you are doing work. You can't just say "I did 10 J of work," it has to be "I did 10 J of work on the object." Then like @scottdave said, you have to take into account all of the forces doing work on the object.

When you lifted the object which system were you doing work on? The object? How about the object-Earth system?
 

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