How Are Potential Energy and Work Related in Physics?

[whitehorsey]

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?

 

[ehild]

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

 

[whitehorsey]

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?

 

[ehild]

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

 

[whitehorsey]

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?

 

[ehild]

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

ehild

 

[whitehorsey]

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

ehild

Ah it becomes positive! So a?

 

[ehild]

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

 

[whitehorsey]

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?

 

[ehild]

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

 

[CAF123]

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?