Register to reply 
Equal but opposite force 
Share this thread: 
#1
Mar214, 01:30 AM

P: 76

If we want to push an electron towards another electron we give equal but opposite force which means we push the electron with the same amount of force that it is being pushed back.
Shouldn't both the forces cancel out the electron remain stationary? 


#2
Mar214, 01:40 AM

P: 8

If you give an equal force to the electron the forces will be neutral so a force greater than the neutral state shall be needed to push



#3
Mar214, 03:18 AM

P: 1

The forces are on two different particles they would only cancel if the two forces acted on the same object in equal magnitude and opposite direction.



#4
Mar214, 03:20 AM

Mentor
P: 3,949

Equal but opposite force



#5
Mar314, 07:31 PM

P: 76

"F in the definition of potential energy is the force exerted by the force field, e.g., gravity, spring force, etc. The potential energy U is equal to the work you must do against that force to move an object from the U=0 reference point to the position r. The force you must exert to move it must be equal but oppositely directed, and that is the source of the negative sign. The force exerted by the force field always tends toward lower energy and will act to reduce the potential energy."  Hyperphysics
The above definition tells us that equal and opposite force must be exerted to move an object against a field. How can the object move? 


#6
Mar314, 08:10 PM

Mentor
P: 17,537

Also, equal and opposite forces on a single object imply no acceleration, not no movement. It will continue moving with it's initial velocity. 


#7
Mar314, 09:24 PM

P: 76

So you mean to say the definition in hyperphysics is wrong?



#8
Mar314, 09:39 PM

Mentor
P: 22,313

No, you are misunderstanding the issue. The issue is that all forces  all forces come in equal and opposite pairs. When you push on a box on the floor and it doesn't move due to friction, that isn't two forces, that's four. Two pair:
You push on the box and it pushes back. The box pushes on the floor and the floor pushes back. See, Newton's 3rd law is talking about force pairs, whereas Newton's second law is talking only about the force applied to the body, not the reaction force applied back. 


#9
Mar314, 09:51 PM

Mentor
P: 17,537

The definition in hyperphysics isn't talking about Newtons third law.



#10
Mar314, 09:52 PM

Mentor
P: 17,537




#11
Mar314, 10:03 PM

Mentor
P: 22,313

The answer is a limiting case of a force negligibly above the weight. For introductory or practical purposes I don't see it as being important to go into that when one first explains it. If someone asks, fine, but if not, it saves a little time. In either case, regardless of the force applied, the force that contributes to the potential energy is exactly equal to the weight. But none of that has anything to do with the original question. The original question was if you push on something and it pushes back with an equal and opposite force, how can it move? That's confusing Newton's 2nd and 3rd laws. 


#12
Mar414, 06:24 AM

Mentor
P: 41,568




#13
Mar414, 07:01 AM

Sci Advisor
HW Helper
P: 6,684

The point that the OP raises is a subtle point often glossed over in texts, as it is in the Hyperphysics statement quoted.
In order to change the position of a charge q at rest in an electric field, E, it is not enough to give it a force equal to qE. That just keeps it in the same position. One has to apply a force to that charge that is greater in magnitude than qE. The application of this force gives q some kinetic energy. A change in potential energy comes from a loss of that kinetic energy when the charge returns to rest in a new position. AM 


#14
Mar414, 07:36 AM

P: 4,214




#15
Mar414, 07:56 AM

Sci Advisor
HW Helper
P: 6,684

AM 


Register to reply 
Related Discussions  
Equal and Opposite Force Help  General Physics  5  
Equal and opposite reactions  General Physics  2  
Equal and opposite?  General Discussion  31 