Conservative Forces: Internal or Not?

In summary, conservative forces are not always taken as internal forces. Gravity is an example of a conservative force, which can be considered internal or external depending on the system boundary. Nonconservative forces, such as friction, can also cause changes in an object's potential energy. The statement that only conservative forces can increase potential energy is incorrect, as demonstrated by the example of lifting an object. In some cases, conservative forces can increase potential energy, as shown by the example of an oscillating puck on an air hockey table.
  • #1
Jon Drake
15
0
Are conservative forces always taken as internal forces?
 
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  • #2
Gravity is a conservative force.
 
  • #3
It's not an internal force, is it?
 
  • #4
What is internal and what is external depends on how you define your system boundary. Gravity is external for many choices, although it could be internal if you include the whole earth.
 
  • #5
Can a nonconservative force cause any change in an object's potential energy?
 
  • #6
Imagine a friction driven lift mechanism. If such a device lifts a load in the gravitational field, it would be changing the potential energy of the object.

What is the point of these questions? Do they relate to anything specific or real?
 
  • #7
I just read a statement somewhere that only conservative forces can increase the potential energy of a system. So I wanted to clarify my doubt.
 
  • #8
Anyway, do conservative forces increase the potential energy? Gravitational force pulls the objects towards the Earth's centre, and that only decreases the gravitational potential energy mgh.
 
  • #9
So, are you now firm in your doubt?
 
  • #10
Yes, and I still cannot understand the logic behind the statement.
 
  • #11
Jon Drake said:
somewhere
Where? It might help if we could see the precise statement and its context.
 
  • #12
Jon Drake said:
Yes, and I still cannot understand the logic behind the statement.
Only conservative forces have a potential energy associated with them. The force acts so as to decrease the potential energy.
 
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  • #13
''Where? It might help if we could see the precise statement and its context.''
It actually came as a question :-
Which of the following can increase the P.E. of a system?
a. Conservative Force
b. Nonconservative Force
c. Both
d. None
The answer was (a).
 
  • #14
Jon Drake said:
''Where? It might help if we could see the precise statement and its context.''
It actually came as a question :-
Which of the following can increase the P.E. of a system?
a. Conservative Force
b. Nonconservative Force
c. Both
d. None
The answer was (a).
That answer is plainly incorrect. If I lift a book from the floor and place it on a table, I have increased the potential energy of the book/earth system. But the force of my hand on the book is not conservative.
 
  • #15
Alright, my final question is, can conservative forces in any case increase the P.E. of a system?
 
  • #16
Jon Drake said:
Alright, my final question is, can conservative forces in any case increase the P.E. of a system?
Set a puck at rest in the middle of an air hockey table. Attach a spring between the puck and a point on one end of the table. Attach a second spring between the puck and a point on the opposite end. Draw your system boundaries so that the first spring and the puck are part of the system but the second spring is not.

Set the puck in motion lengthwise along the table and watch it oscillate.

The first spring embodies a conservative force field. So does the second one. The conservative force from the second spring is an external force that is alternately doing work on the system, increasing its energy and extracting work from the system, decreasing its energy. At the halfway point in the oscillation, the puck is neither accelerating nor decelerating. Kinetic energy is unchanging and we have what is undeniably a change in potential energy caused by a conservative force.
 
  • #17
Yes, you are right. I now understand. Thanks for the explanation.
 

1. What are conservative forces?

Conservative forces are a type of force that depends only on the initial and final positions of an object, and not on the path taken between them. These forces are always internal to a system and do not cause any change in the total mechanical energy of the system.

2. How can I identify if a force is conservative or not?

A force is conservative if it meets two criteria: it is an internal force and its work is independent of the path taken. This means that if you move an object from one point to another, the work done by the force will be the same no matter which path is taken between the two points.

3. Are all internal forces conservative?

No, not all internal forces are conservative. Only forces that meet the criteria of being independent of the path taken and not causing a change in mechanical energy are considered conservative. For example, friction is an internal force that is not conservative as it dissipates energy in the form of heat.

4. How do conservative forces affect the motion of an object?

Conservative forces do not affect the motion of an object in any way. This is because they do not cause any change in the total mechanical energy of the system. They only dictate the potential energy of an object at different positions.

5. Can conservative forces do work?

No, conservative forces do not do any work on an object. This is because work is defined as a force acting on an object over a certain distance, and since conservative forces do not cause any change in the total mechanical energy, they do not do any work on the object.

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