How Are Potential Energy and Work Related in Physics?

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Homework Help Overview

The discussion revolves around the relationship between potential energy and work in the context of gravitational forces, particularly focusing on the Earth-object system. Participants explore the implications of potential energy changes as an object is moved in a gravitational field.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the sign of potential energy and its implications, questioning whether potential energy can be negative or positive based on the reference point chosen. There are inquiries about how potential energy changes as an object moves further from the Earth and how this relates to work done against gravitational forces.

Discussion Status

The discussion is active, with participants exploring various interpretations of potential energy and its relationship to work. Some guidance has been offered regarding the calculations and concepts involved, but there is no explicit consensus on the interpretations being discussed.

Contextual Notes

Participants are navigating assumptions about reference points for potential energy and the definitions of work in the context of gravitational forces. There is an ongoing exploration of the implications of negative values in potential energy calculations.

whitehorsey
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1. When taking an object and the Earth as a system, its change in potential energy is given by
a. m*g*change in height
b. (-m)g*change in height
c. either a or b
d. there is not potential energy due to gravity


3. I think it is c because the sign of energy does not matter?
 
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The sign does count. Is the potential energy of the Earth-object system negative or positive if you take the zero of potential energy at infinite separation?

ehild
 
ehild said:
The sign does count. Is the potential energy of the Earth-object system negative or positive if you take the zero of potential energy at infinite separation?

ehild

What's infinite separation?
 
I meant infinite distance between the centre of Earth and the object where the potential energy of the whole system is zero. The force between them is GmM(earth)/r2, towards each other. That corresponds to the potential energy of interaction U=-GmM/r. Does the potential energy increase or decrease if the object gets farther from the Earth centre?

ehild
 
ehild said:
I meant infinite distance between the centre of Earth and the object where the potential energy of the whole system is zero. The force between them is GmM(earth)/r2, towards each other. That corresponds to the potential energy of interaction U=-GmM/r. Does the potential energy increase or decrease if the object gets farther from the Earth centre?

ehild

Oh! Decrease?
 
Oh not! The magnitude of a negative value decreases, what does it mean?

ehild
 
ehild said:
Oh not! The magnitude of a negative value decreases, what does it mean?

ehild

Ah it becomes positive! So a?
 
whitehorsey said:
Ah it becomes positive! So a?

more positive.

Near the Earth surface, the force on the object is mg (downward). Does the potential energy increase or decrease if the object raises by h?


ehild
 
ehild said:
more positive.

Near the Earth surface, the force on the object is mg (downward). Does the potential energy increase or decrease if the object raises by h? ehild
PE = (-mg)*(+h) = -mg. So PE decreases too which leads to a?
 
Last edited:
  • #10
whitehorsey said:
PE = (-mg)*(+h) = -mg. So PE decreases too which leads to a?

No, the potential energy is the work against the force of gravity... The work needed to lift the object...

ehild
 
  • #11
whitehorsey said:
PE = (-mg)*(+h) = -mg. So PE decreases too which leads to a?

You have found the work done by the force of gravity as the object is moved a height h. How are potential energy and work related?
 

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