Work Against Gravity: Understanding Force and Energy

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Discussion Overview

The discussion revolves around the concept of work done against gravity when lifting an object, specifically a 1.0 kg ball, to a height of 1.0 meter. Participants explore the relationship between the force applied, the work done, and the gravitational potential energy (GPE) of the object, including scenarios where the applied force exceeds the weight of the object.

Discussion Character

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

Main Points Raised

  • One participant states that lifting a 1.0 kg ball to a height of 1.0 meter against gravity requires about 10 joules of work, which corresponds to the gravitational potential energy (GPE) at that height.
  • Another participant clarifies that lifting the ball with a force of 15 Newtons results in 15 joules of work, indicating that this is more than the minimum required to lift the ball, suggesting that the excess work contributes to kinetic energy (KE).
  • There is a contention regarding the interpretation of the work-energy theorem, with one participant asserting that the theorem states the work done must equal the change in energy of the object, which includes both GPE and KE.
  • One participant acknowledges the clarification regarding the relationship between the force applied and the work done, indicating a better understanding of the concepts discussed.

Areas of Agreement / Disagreement

Participants express differing views on the implications of lifting an object with a force greater than its weight, particularly regarding the distribution of work done between gravitational potential energy and kinetic energy. The discussion remains unresolved as participants have not reached a consensus on the interpretation of the work-energy theorem in this context.

Contextual Notes

There are unresolved assumptions regarding the definitions of work, energy, and the conditions under which the work-energy theorem applies. The discussion does not clarify the implications of lifting with varying forces on the overall energy state of the ball.

jldibble
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I'm figuring that this has been asked before, but I couldn't locate a previous thread.

Here's my problem:

A 1.0 kg ball is lifted at a constant speed to a height of 1.0 meter above the ground. The work done to the ball against gravity is about 10 joules.

The same 1.0 kg ball is lifted with a force of 15 Newtons to a height of 1.0 meter above the ground. The work done to the ball in this case is 15 joules.


What is getting me confused is that the work done to lift the ball 1.0 meter should be the same no matter what. And the work energy theorem says that a 1.0 kg object 1.0 meter above the ground should have a GPE of 10 joules. So my problem is with lifting objects with a force greater than their weight.

What am I getting mixed up here?
 
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jldibble said:
A 1.0 kg ball is lifted at a constant speed to a height of 1.0 meter above the ground. The work done to the ball against gravity is about 10 joules.
That's the minimum amount of work you need to do to just lift the ball against gravity.

The same 1.0 kg ball is lifted with a force of 15 Newtons to a height of 1.0 meter above the ground. The work done to the ball in this case is 15 joules.
Here you did more work that necessary to lift the ball. So that extra work goes into the increased KE of the ball.

What is getting me confused is that the work done to lift the ball 1.0 meter should be the same no matter what.
No, not really.

And the work energy theorem says that a 1.0 kg object 1.0 meter above the ground should have a GPE of 10 joules.
No, the work energy theorem just says that the work you do must equal the change in energy of the object. And it does!
 
You are correct, the 15 N force will result in 15 J of work done. At the end the ball will have 10 J of GPE and 5 J of KE.
 
Oh, of course... Easy enough. Thanks!
 

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