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Has anybody come across the term "real force" before? My textbook defines it as follows:

"A real force is a force which acts on an object due to another object. An isolated object(far from all objects doesn't experience any real force."

What must I make of this absurd definition?

Qmechanic
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    Hey, I've answered a lot of these kinds of questions from your peers, and I have to say, this is absolutely not your fault. The textbook really doesn't make any sense! This seems to be a really common problem with Indian physics textbooks, I don't know why. – knzhou Aug 24 '16 at 02:39
  • The only advice I have to give is, please consider buying a standard book from the U.S., such as Halliday/Resnick/Krane or Giancoli or Young and Freedman. These are written in a much more friendly way. Or, try online lectures, like on EdX, KhanAcademy, or Coursera. – knzhou Aug 24 '16 at 02:40
  • That's just gibberish. – garyp Aug 24 '16 at 02:41
  • I think they mean kinematic force, "classical mechanical" understanding of force. There is not such thing as real force. – MsTais Aug 24 '16 at 02:41
  • @knzhou Unfortunately, the textbooks in India are State or Central Board published books. And in exams, the same definitions and explanations given in those books are expected. Otherwise, marks are not given in spite of providing correct answers. – Apoorv Potnis Dec 03 '17 at 15:50

4 Answers4

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A real force is a force with reaction force, which satisfies Newton's third law. Fictitious forces have no reactions.

velut luna
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  • Newton's third law, in the limited sense of pairs of forces can be violated by any macroscopic force, both "real" and "fictitious" (like gravity, electromagnetism, etc..). In the general sense of conservation of momentum though the 3rd law still holds both for "real" and "fictitious" forces (see related answer). So this answer is not accurate. – Nikos M. Mar 04 '24 at 10:51
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See the comment... what you should understand though in that respect is that forces always have their agents.

Dynamic force - physical bodies, ropes, bullets, whatever you encounter in your early undergraduate physics life

Dissipative (friction, tension...) - complicated... can be phonons, plasmons, molecular vibrations etc.

Electromagnetic - photons

and so on...

You can classify them in any way you want... But what the textbook calls real is rather dynamic. I have never heard of "real" forces, lol... real forces, real dudes=)))

MsTais
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You are right: that proposition is a mess but we can try to to shape it up to make some sense.

I suppose you know the difference between a real force and an unreal or fictitious force. In case you do not know, the latter is one of the many artifices of ancient physicists who tried to deal with phenomena they did not fully understand. A fictitious force is what a naive observer would suppose to explain a motion which is not real, but which depends on the frame of observation. If it is not clear and you never heard, for example, of Coriolis force, think of planets that appear to move in a backward direction, or the moon itself or the Sun that seems to move around the earth.

A real force is a source of energy, and more precisely of kinetic energy. That can be a charge attracting or repelling other bodies, or the muscle of you body expanding and setting a ball in motion.

That was the cryptic meaning ot your book:

  • the abstract 'real force'
  • in a real object (your muscle) acts
  • on another object (the ball)
  • due to the other object (your physical muscle)
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    This and Serge Ballesta's answers are the only useful answers I've gotten so far. Thanks :) –  Sep 16 '16 at 12:39
  • Inertial forces originating in relative motion of frame of reference is and feels as "real" as any force can be (put yourself in a rotating room and find out). – Nikos M. Mar 04 '24 at 19:38
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A good example of a non real force is the well known centrifugal force. It is only an apparent force that acts on a mass in a circular rotation movement (for example). Using this apparent force allows to process simple mechanics operation relative to moving mass as if it did not move.

Serge Ballesta
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  • This and user104372's answers are the only useful answers I've gotten so far. Thanks :) –  Sep 16 '16 at 12:38