# What is a force?

1. Jun 10, 2004

### Michael F. Dmitriyev

A concept "force" is known thousand years. It defines a certain essence capable to make a work i.e. to carry out the moving of objects or on a contrary to prevent this moving.
And always a force is directed on overcoming of other force. All properties of forces are described by sir Isaac Newton. But the question “ What is a force? ” remains open on today.
So. What is it?

2. Jun 10, 2004

### JohnDubYa

Any phenomenon that attempts to change the motion of a body.

I would not define force in terms of work. For example, the normal force (a non-conservative force) acts on a roller coaster throughout its motion, but does no work.

3. Jun 10, 2004

### Gza

Force is an idea in our heads. The funny thing about this idea, is that it tends to make nature easy to predict. The idea is that if we see an object change it's inertial state of motion, we explain this occurence by a force.

4. Jun 10, 2004

### Michael F. Dmitriyev

You are right. A force attempts to change the motion of body. But, is it possible to equal phenomenon with a force? A phenomenon is the source of force. It does not explain how this force move an object at microlevel.

5. Jun 10, 2004

### JohnDubYa

Well, you are asking "What is an interaction?" That is like trying to define time.

Even ordinary objects are hard to describe. What really is an apple? You can say it is a type of sweet fruit, but then what does "sweet" mean? And on, and on, and on.

So I guess my question is, what is the point of this discussion?

6. Jun 11, 2004

### pmb_phy

Jammer touched on this in his book Concepts of Force. From page 124
In modern physics force is defined as the time rate of change of momentum. In Newton's day that was a law of physics rather than a definition of force.

Pete

Last edited: Jun 11, 2004
7. Jun 11, 2004

### JohnDubYa

RE: "In modern physics force is defined as the time rate of change of momentum. "

No, that would be the NET force.

Defining force in terms of momentum change is problematic because it has the cause/effect relationship backwards. Net forces cause momenta to change, not the other way around.

8. Jun 11, 2004

### pmb_phy

The distinction isn't really a biggy since "force" should always mean "sum of forces" i.e. net force. Most people abuse the concept and term "force". E.g. consider a charged particle in the presence of two other particles. The time rate of change of momentum of the charged particle will equal the force acting on the particle. The force is the superposition of two other forces. What that literally means is that the force on the particle equals the force due to particle one in the absense of all other forces/particles + the force due to particle two in the absense of all other forces/particles. Call that "Force" or "net force" or whatever you like. To phrase this poorly (as it usually is) the net force on the charge is the sum of the forces of the other two particles. The former description is rigorous, the later is sloppy.
You're using the term "force" as a cause. Force is correctly defined as the time rate of change of momentum.

Sorry but I don't see any problem of cause/effect.

Pete

Last edited: Jun 11, 2004
9. Jun 11, 2004

### JohnDubYa

RE: " The force is the superposition of two other forces."

That makes zero sense.

Consider an apple resting on a table.

"The apple is not accelerating, so there is no force acting on the apple."

"Yes, there is a force of gravity acting on the apple."

"So there IS a force acting on the apple?"

"No."

"Waaaaaaaaaahhhhh Physics sucks! I'll never understand this freakin' subject!"

The proper term is "The vector sum of all forces acting on the object..." That is completely unambiguous and correctly describes the mathematical relationship between the forces. Students won't cry when we describe forces in this manner.

Forces are the causes, the change in momentum is the effect. You cannot define a cause by simply defining the effect.

10. Jun 12, 2004

### pmb_phy

What about that statement don't you understand? Let me elaborate. Let there be 3 charges q1, q2, and q3. Let charges q2 and q3 be fixed.

Let F21 be the force which would be exerted on charge q1 by charge q2 if q3 was not present.

Let F31 be the force which would be exerted on charge q1 by charge q3 if q2 was not present.

The force (aka total force or what you call "net force") F on the particle when both charges are present is given by

F = F21 + F31

Regarding the apple. The force on the apple is zero. This is the same kind of thing above. It just isn't as clear or as easy to describe as the example above. I didn't say that this was easy for students to grasp. I said it was rigorously true. Also it's inappropriate to call force a cause. At least in the way force is defined in modern physics. For a Newtonian that might be a good way to describe it.

For the ambitious - See

On force and the inertial frame, Robert Brehme, Am. J. Phys., Vol. 53(10), Oct 1985

Feynman Lectures, V-II. See section 12-1 which is labeled What is force?

As Feynman says "If you insist on a precise definition of force, you will never get one."

Pete

Last edited: Jun 12, 2004
11. Jun 12, 2004

### dedaNoe

Force is primary defined as magnitude that describes the mutual interaction between at least two bodies.
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In my personal experience force is geometric potential.
Force is storage for the distance from the equilibrium point that is yet to be achieved.
How much force you have that much distant you'll become from your initial equilibrium state.

12. Jun 12, 2004

### pmb_phy

That can get circular since, to be precise, one has to now define "interaction".

Pete

13. Jun 12, 2004

### Staff: Mentor

dedaNoe,
If you wish to discuss your personal theories, please do it in the Theory Development forum, not here.

14. Jun 12, 2004

### JohnDubYa

RE: "Regarding the apple. The force on the apple is zero. This is the same kind of thing above. It just isn't as clear or as easy to describe as the example above."

Which means the definition is faulty.

We are free to define the words in any manner we wish. Giving the individual forces the same label as the net force causes confusion. Therefore such a definition is faulty.

RE: " I didn't say that this was easy for students to grasp."

Right there that should tell you something is wrong. Students can easily grasp the notion of "the vector sum of all forces acting on an object." And since that statement makes perfect sense and is mathematically correct, use it.

You gain nothing by calling the vector sum of all forces the "force." You only increase confusion. In science we try to clarify, not obfuscate.

But to further the argument along, suppose I decided to call the sum of all individual forces the "net force." Prove that such a definition is incorrect by showing me an instance where such a definition predicts the wrong observed behavior.

RE: "I didn't say that this was easy for students to grasp. I said it was rigorously true."

Define "true." What do you mean by "true" when we are free to define the net force in any manner we wish? If we decided to call the net force the "widget," that would be no different than if we called it the "force."

RE: "Also it's inappropriate to call force a cause."

Well SOMETHING caused the motion. So if it isn't the force, what is it?

By the way, I don't give a crap what Feynman thinks. He has his opinions and I am free to disagree with him. (I don't in this instance, by the way.)

15. Jun 13, 2004

### Michael F. Dmitriyev

Does it means that all attempts of a precise definition of force are forbidden forever?

16. Jun 13, 2004

### JohnDubYa

Yep, because FEYNMAN SAYS SO!

17. Jun 14, 2004

### Michael F. Dmitriyev

There is no God except of a God. I hope that in a case with force FEYNMAN was mistaken.

18. Jun 16, 2004

### Michael F. Dmitriyev

At absence of any idea, a following one can be useful.
If mass does act at the space-time (Einstein), then space-time does act at mass (Newton’s III).

19. Jun 17, 2004

### Imparcticle

Is it possible to conclude that many if not all definitions of something do not give a precise definition but one that describes it in an abstract sense?

20. Jun 17, 2004

### Michael F. Dmitriyev

Is it not a purpose of science the transition from an abstract description to precise definition?