Work, Energy & Friction: How Do They Fit?

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Discussion Overview

The discussion revolves around the concepts of work, energy, and friction in the context of lifting and dragging a ball. Participants explore the relationships between applied forces, resultant forces, potential energy, kinetic energy, and the effects of friction on mechanical energy. The scope includes theoretical reasoning and mathematical calculations related to these concepts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant states that lifting a 2kg ball with a force of 25N results in a total work of 125J, with 100J converted to potential energy and 25J to kinetic energy.
  • Another participant questions the use of resultant force in calculating work done against friction and suggests that the work done against friction should be considered in the calculations.
  • Some participants discuss the distinction between conservative forces (like gravity) and non-conservative forces (like friction), noting that energy from friction is converted to heat and not added to mechanical energy.
  • There is a suggestion that the work done against friction should be included in the total work calculation when dragging the ball along the floor.
  • One participant expresses confusion about the concept of "extra mechanical energy" and its implications in the calculations.

Areas of Agreement / Disagreement

Participants express differing views on how to calculate work done in scenarios involving lifting and dragging objects, particularly regarding the treatment of friction and resultant forces. The discussion remains unresolved, with multiple competing views presented.

Contextual Notes

Some participants rely on specific definitions and interpretations of work and energy that may not be universally agreed upon. There are unresolved mathematical steps and assumptions regarding the forces at play in each scenario.

c77793
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I've always seen Work as an extra mechanic energy given to the system... but i really can't figure it out :

If we lift a 2kg ball, that's steady, using g as 10m/s², with 25N, we'll have a 5N up resultant force...

after its 5meters up, the resultant work will be 25J (5N.5m), but the potential energy by itself will be 100J (20kgx10m/s²x5m) and the kinetic energy will be 25J (i found it using the F=ma to discover de aceleration and then using torricelli's formula..)..

How can I give 25J to a system and it become 125J?

If the same ball were dragged along the floor, with 25N, and a friction force of 20N , after 5 meters, the resultant work would be 25J, the same as the extra mechanic energy ... can someone help me?
 
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you still used 25N to lift, so the work done by you would be:

25 N*5 m = 125 J

From that 125 J of work, 100 was stored as potential energy, the other 25 is being used as kinetic energy (since the object is still moving).

If you are dragging a ball against friction, you are doing work against the friction, so the work is 45 N * 5 m = 225 J.

I don't know what "extra mechanical energy" means.
 
But shouldn't I use the Resultant force?

as if it were horizontally and the Gravity force were the friction force?
 
well, the friction will do a work of -100J (once its againts the movement) and the force will do a 125 work... the work will be 25, as if it were a 5N force only... I learned that way, is it wrong?
 
c77793 said:
But shouldn't I use the Resultant force?

as if it were horizontally and the Gravity force were the friction force?

It depends on the exact question. If you're asking how much work you do to pull it along the ground then you are doing work against friction, so that must be added.
 
c77793 said:
well, the friction will do a work of -100J (once its againts the movement) and the force will do a 125 work... the work will be 25, as if it were a 5N force only... I learned that way, is it wrong?

Well no, the wording you've used here makes it right. You specified which force did how much work and used the qualifier "as if". So your statements here are correct.
 
I think I got it... when it comes to friction, the energy becomes heat, so we just cut it off the equation to calculate the Mechanical Energy...

but when it comes to Gravity force, the energy becomes potential energy, so we add it to the equation... is it right?
 
c77793 said:
I think I got it... when it comes to friction, the energy becomes heat, so we just cut it off the equation to calculate the Mechanical Energy...

but when it comes to Gravity force, the energy becomes potential energy, so we add it to the equation... is it right?

Yes, that's true. Gravity is a conservative force, friction is not.
 

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