## can someone explain me why f=ma ?

hey guys i had a doubt can someone please explain me why force=mass x acceleration?
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 Recognitions: Gold Member That's how the concept of force is defined. If you apply larger amounts of net force on an object, you'll find that the acceleration increases linearly and the property that decides how quickly the acceleration increases is the mass.
 A quicky my teacher once said: " I don't like the way the original equation is written because it brings up misconceptions to many new students, instead think of it this way, f/m=a. A force acting on a body will cause it to accelerate". Then you only also have to remember one of the laws of newton: that a body will stay at rest (or uniform motion in a straight line) as long as no net force is applied to the body. And when suddenly we have a net force, we have an acceleration. Not an expert in this subject but maybe that will help. Best Regards Robin Andersson

## can someone explain me why f=ma ?

There's a fun quote by Arthur Eddington which touches on this:
 Every body continues in its state of rest or uniform motion in a straight line, except insofar as it doesn't.
F=ma is sometimes looked at as a definition (especially by mathematicians), where it constitutes the definition of force based on measurable quantities mass and acceleration. But according to Thornton and Marion, really when Newton stated this, he meant something physical. I think what Newton really meant, more specifically, was that from an inertial reference frame, objects continue in straight lines unless acted upon by a net force--but the concept of an inertial reference frame isn't defined when you begin to state Newton's laws--so it's kind of a built-in assumption that's hard to avoid. That's why the mathematicians just ignore the physical meaning and take it as a definition.
 Mentor Usually Newton's first law is considered to be a definition of inertial frames and Newton's second law is considered to be a definition of forces. Once those terms are defined, then Newton's third law is the one that contains the actual physics.
 I beg to differ, DaleSpam. Newton's third law applies only to interactions between two (or more) bodies--if that law contained all the actual physics, then how is it possible that there is a whole class of problems that deal with a single particle under the influence of some "external" force or field? For example, a particle on a spring (SHO), a massive particle in an external gravitational field, and a charged particle in an EM field all are nontrivial physical problems which can be solved using only the first two laws--not the third.
 Mentor Sure, but in all of those cases there is something besides Newtons three laws defining the physics, the force law. You mentioned Hookes law, Newtons law of gravitation, and Maxwells equations. Newtons first two laws are still generally considered to be definitions in the scenarios you mentioned.
 Well, Hooke's law isn't exactly a physical law of the universe--it just is a force function. You could treat Newton's gravitational force law as just some random "given" force with a certain functional form, and likewise, you could ignore Maxwell's equations and simply give the Lorentz force law. If you want some even more trivial examples, you could have F=constant, F(t)=at+b, F(x) = cos(x), or whatever. They all are just given "external" forces. All problems of this form do not require the third law.
 Mentor Agreed, but again, the physics is not contained in the first two laws, they are simply definitions. The physics is contained in whatever force law you propose or in the third law. All problems involving only the first two laws are simply problems designed to help students learn the definitions.

Well, I believe we are still at odds here. I think the problems which involve only the first two laws do contain actual physics--they tell you how a particle actually moves under the influence of a force. And though many of the examples I gave do seem like problems you'd only give to help students learn the definitions, I think the simple harmonic oscillator and a particle under the Lorentz force law are quite a bit beyond just learning the definitions. They give nontrivial predictions on trajectories.

Even in well-respected textbooks, you'll find admissions that the scenario is not as clear-cut as what you've argued. There are alternate ways of answering the question, and they amount to different perspectives on these laws.

Thornton and Marion's "Classical Dynamics of Particles and Systems," Fifth Edition, treats this question with a few paragraphs (pp.49-50), and it includes this footnote on page 50.
 The reasoning presented here, viz., that the First and Second Laws are actually definitions and that the Third Law contains the physics, is not the only possible interpretation. Lindsay and Margenau (Li36), for example, present the first two Laws as physical laws and then derive the Third Law as a consequence.
Your view is certainly a valid way of looking at things, but it is not the only possible way.
 I'm sure the answer is that it's an empirical law. If you apply double the force, you observe the object accelerates twice as much. Do this experiment in enough ways and it's soon fairly convincing that F=ma and not m/a or ma^2 or something weird.

 Quote by DaleSpam Usually Newton's first law is considered to be a definition of inertial frames and Newton's second law is considered to be a definition of forces. Once those terms are defined, then Newton's third law is the one that contains the actual physics.
This is not how Newton gave them. See http://en.wikisource.org/wiki/The_Ma...Laws_of_Motion

"LAW II.

The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.

If any force generates a motion, a double force will generate double the motion, a triple force triple the motion, whether that force be impressed altogether and at once, or gradually and successively. And this motion (being always directed the same way with the generating force), if the body moved before, is added to or subducted from the former motion, according as they directly conspire with or are directly contrary to each other; or obliquely joined, when they are oblique, so as to produce a new motion compounded from the determination of both."

Definitions were given before the laws:

http://en.wikisource.org/wiki/The_Ma...6)/Definitions

"DEFINITION IV.

An impressed force is an action exerted upon a body, in order to change its state, either of rest, or of moving uniformly forward in a right line."

It can also be seen that his definition of force refers to inertial frames, but perhaps not entirely satisfactorily (the translator may be to blame, though).

 Quote by MikeyW I'm sure the answer is that it's an empirical law. If you apply double the force, you observe the object accelerates twice as much. Do this experiment in enough ways and it's soon fairly convincing that F=ma and not m/a or ma^2 or something weird.
This repeats Newton's own words almost verbatim! In the scholium that follows the Laws, he refers to multiple experiments, particularly by Galileo, that had established these laws.

Mentor
 Quote by voko This is not how Newton gave them.
Correct, it is not how Newton gave them. It is how modern physicists interpret them with the advantage of several hundred years of hindsight.

Mentor
 Quote by Jolb Your view is certainly a valid way of looking at things
Excellent, then it seems we are not at odds.

 Quote by Jolb but it is not the only possible way.
Which is why I qualified my statements with the word "usually".

Btw, the reason I prefer the usual approach over alternatives is the difficulty in defining what a force is without using f=ma. It can be done, usually via an experimental prototype force, but force prototypes are not very reproducible.
 Actually, it is an 'observation' .. not really a definition: and the observation is simply : what features of an object are directly proportional to the 'force needed to move it'.... and what features, if any, are found to be INVERSEly proportional to that force ... So, experiment shows that only two items affect the ability of a force in a friction-less vacuum to move any object ... and both of them are DIRECTLY proportional to the force required ... Ie, if the MASS is bigger, the force must be bigger .. and if the required motion is to start, then the force also is directly proportional to how fast the mass is made to move up to any velocity Mathematicly, we state such a condition this way: In english: for a given force, giving it a velocity is DIRECTLY proportional to the mass of the object, and also Directly proportional to how much you choose to accellerate it. ... In math: F = M TIMES any other 'directly propportional' item .. in this case, 'A' , how much we choose to make it go faster.... with a 'Constant of Proportionality' < 'K'> conveniently chosen to be '1' , or: F = kMA , where k = 1 for our chosen units If there were any 'INVERSELY Proportional entity, say 1/Z , or: F=kMA/Z because the BIGGER Z is, the SMALLER F can be ... ie, they are INVERSELY proportional. This is how many relationships are described ... another example woujld be 'Ohm's Law' where: E = I x R .. I'll leave that to yu to sort out ;) and remeber: I MUST = E/R , atain from the observation.

 Quote by MikeyW I'm sure the answer is that it's an empirical law. If you apply double the force, you observe the object accelerates twice as much. [... ]
The problem is: How do we know that we've doubled the force? How else do we measure the force except using $F = kma$ itself? In other words, we're brought back to regarding $F = kma$ as true by definition.

I've just been a bit of a devil's advocate, though, because things aren't quite as simple as I've made out. For example, if we use two identical, equally stretched springs, pulling in the same direction on an object, instead of just one of the springs, we find twice the acceleration (if resistive forces are negligible). It's hard to quarrel with the idea that doubling the springs must double the force (so giving us an independent way of knowing that we've doubled the force), but I've known people who believe that it's making an unacceptable assumption.

Mentor
 Quote by Philip Wood For example, if we use two identical, equally stretched springs
Yes, this is exactly what I meant by "an experimental prototype force".

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