# B Conservative Forces

1. Dec 2, 2017

### Jon Drake

Are conservative forces always taken as internal forces?

2. Dec 2, 2017

### Dr.D

Gravity is a conservative force.

3. Dec 2, 2017

### Jon Drake

It's not an internal force, is it?

4. Dec 2, 2017

### Dr.D

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. Dec 2, 2017

### Jon Drake

Can a nonconservative force cause any change in an object's potential energy?

6. Dec 2, 2017

### Dr.D

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. Dec 2, 2017

### Jon Drake

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. Dec 2, 2017

### Jon Drake

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. Dec 2, 2017

### Dr.D

So, are you now firm in your doubt?

10. Dec 2, 2017

### Jon Drake

Yes, and I still cannot understand the logic behind the statement.

11. Dec 2, 2017

### Staff: Mentor

Where? It might help if we could see the precise statement and its context.

12. Dec 2, 2017

### PeroK

Only conservative forces have a potential energy associated with them. The force acts so as to decrease the potential energy.

13. Dec 2, 2017

### Jon Drake

''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

14. Dec 2, 2017

### jbriggs444

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. Dec 2, 2017

### Jon Drake

Alright, my final question is, can conservative forces in any case increase the P.E. of a system?

16. Dec 2, 2017

### jbriggs444

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. Dec 2, 2017

### Jon Drake

Yes, you are right. I now understand. Thanks for the explanation.