What Is a Force? Definition & Explanation

[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?

 

[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.

 

[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.

 

[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.

 

[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?

 

[pmb_phy]

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?

Jammer touched on this in his book Concepts of Force. From page 124

Force, for Newton, was a concept given a priori, intuitively, and ultimately in analogy to human muscular force.

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

 

[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.

 

[pmb_phy]

No, that would be the NET force.

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.

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.

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

 

[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."

"What about gravity"

"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.

 

[pmb_phy]

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

That makes zero sense.

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

 

[dedaNoe]

Force is primary defined as magnitude that describes the mutual interaction between at least two bodies.
***************************
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.

 

[pmb_phy]

Force is primary defined as magnitude that describes the mutual interaction between at least two bodies.
***************************
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.

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

Pete

 

[Doc Al]

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.

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

 

[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.)

 

[Michael F. Dmitriyev]

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

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

 

[JohnDubYa]

Yep, because FEYNMAN SAYS SO!

 

[Michael F. Dmitriyev]

Yep, because FEYNMAN SAYS SO!

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

 

[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).

 

[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?

 

[Michael F. Dmitriyev]

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?

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

 

[Imparcticle]

Well apparently, we have no precise definition of what a force is, but rather we have a definition dependent on the influence of a force. For example, according to the definition given by the Encyclopedia Britannica, (http://www.britannica.com/ebc/article?eu=389981&query=force&ct= )
"force" is an action...but an action comming from what? What is the source of the influence?

 

[Gza]

then space-time does act at mass (Newton’s III).

I highly doubt Newton had any idea of what space-time is.

 

[Michael F. Dmitriyev]

I highly doubt Newton had any idea of what space-time is.

Probably, but nobody canceled the Newton’s third law on today. So, any action (force) has equal opposite one.
Hence the space-time bending caused by mass does cause an equivalent action at mass.

 

[JohnDubYa]

RE: " Probably, but nobody canceled the Newton’s third law on today. So, any action (force) has equal opposite one."

Hmmm... Suppose I have two electrons, A and B. One traveling North at 1000 m/s and the other traveling East at 1000 m/s.

When electron A crosses the path of electron B, what is the magnitude of the force on each (assume electron A crosses 1 meter behind electron B)?

 

[arivero]

A concept "force" is known thousand years.

Are you sure? Is the English concept "force" exactly equal to the spanish concept "fuerza" or to the Italian concept "forza"? I have heard football teams crying "forza Italia!", but I can not imagine "force England!".

 

[Michael F. Dmitriyev]

Well apparently, we have no precise definition of what a force is, but rather we have a definition dependent on the influence of a force. For example, according to the definition given by the Encyclopedia Britannica, (http://www.britannica.com/ebc/article?eu=389981&query=force&ct= )
"force" is an action...but an action comming from what? What is the source of the influence?

Certainly force is an action and for objects it means a change in space and time. Though, how to define force when spoken about force of mind or of public opinion, for example? Or it is inappropriate (wrong) applications of the term “force”?

 

[Michael F. Dmitriyev]

Are you sure? Is the English concept "force" exactly equal to the spanish concept "fuerza" or to the Italian concept "forza"? I have heard football teams crying "forza Italia!", but I can not imagine "force England!".

I guess, an analogues of words "force" or "fuerza" or "forza" are exist in other languages too.
And, I guess also, they were applied in the more ancient games than football invented in England not so long time ago.

 

[Michael F. Dmitriyev]

RE: " Probably, but nobody canceled the Newton’s third law on today. So, any action (force) has equal opposite one."

Hmmm... Suppose I have two electrons, A and B. One traveling North at 1000 m/s and the other traveling East at 1000 m/s.

When electron A crosses the path of electron B, what is the magnitude of the force on each (assume electron A crosses 1 meter behind electron B)?

For the exact decision of this task it is necessary to examine interaction of these two objects not only in a point of crossing, but also on a big enough way of their travel before and after this point. This task of dynamic interaction can be solved successfully with the computer’s help. It is just possible to say definitely now - in a result of this interaction the direction of their traveling will not be strictly North and East (at a moment of crossing of their paths).

 

[Michael F. Dmitriyev]

The truth is, we can never ever understand anything completely.

Our mind uses emotions which we normally experience within nature to put these type of things into perspective.

So, we should all use an idea which works best for us. There are some emotions which cannot fit the concept of certain things, which is why very few of us can understand the concept of dimensions.

In general, you are right. But any reasons about hopelessness of attempts to know all secrets of the nature cannot stop these attempts. Probably it is incorporated in the person initially and has practical sense for the nature itself.

 

[Macca604]

Yes, I agree.

 

[Michael F. Dmitriyev]

So, we know a force is action.
If some object is subjected to some action we always (?) know a source this action. We have a force source (an object 1), the receiver of force ( an object 2) and the result. Obviously here there is no some carrier of force.
The arrow, which designate usually a vector of force, does not carry out this function.
The explanation of action through nothing (at a microlevel) by means of another nothing, can not convinces at all.
What is a carrier of force?

 

[Michael F. Dmitriyev]

There may be, the some known properties of force can lead us to the carrier of this force.
The vector of force is a direct line i.e. the carrier of force always propagates on a direct line too.
Force itself has no mass or its mass is close to zero. Hence the carrier of force cannot have mass too.
These properties are amazingly similar to the properties of light.
Is it not so?

 

[Michael F. Dmitriyev]

So, we can quite definitely say:
- a force this an action of one object at another one by means of light, which is a carrier of this action.

 

[Moe]

Force doesn't have a mass. The particle that propagates it might have a mass in some cases. And a photon isn't always the propagator of a force. For EM it is, but for the strong force there are 8 gluons IIRC, for the weak force you have 3 different bosons, and for the gravitational force, well, that thingy hasn't been discovered. Yet.

 

[Michael F. Dmitriyev]

Force doesn't have a mass. The particle that propagates it might have a mass in some cases. And a photon isn't always the propagator of a force. For EM it is, but for the strong force there are 8 gluons IIRC, for the weak force you have 3 different bosons, and for the gravitational force, well, that thingy hasn't been discovered. Yet.

Moe,
You are completely right if to accept a force as action at objects.
Let to look at force from another point of view.
Result of action of force is a change in SPACE.
This change is absolute but has appeared in such relative things as speed of an object, its temperature, pressure and all other displays of action of a force. Hence we can say:
- force is an action at space between objects.
Further. What constantly radiate and absorb particles and atoms?