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Inertia..is what?

  1. Oct 25, 2015 #1
    Hello. Inertia is the resistance a material object has to a change in it's present motion because of an applied force..or even if an object is initially motionless.

    Why does matter have this quality? In the vacuum of outer space should it require virtually no energy to impart momentum to matter? What is causing the resistance?

  2. jcsd
  3. Oct 25, 2015 #2

    Simon Bridge

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    Nobody knows. Physics is not good at this sort of "why" question - we simply know that it does.
    No. There is no "should" about it - we know that it does require work to accelerate a mass.

    Nobody knows. Inertia - which is to say, mass, is a fundamental property that many objects have. If you were to find that inertia were caused by something else more fundamental then you'd just be faced with wondering where that property came from and why it imparts inertia.
  4. Oct 25, 2015 #3


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    Because that's how matter behaves in the universe that we live in.

    I know, that's not a very satisfying answer, but it's all there is. Physics is about describing the universe and its rules of operation. It's not so good at explaining why the rules are what they are and not something different.

    You can explain inertia in terms of conservation of momentum, but that just moves the "because it is" answer around. Why is momentum conserved in the universe we live in? Noether's theorem says that it follows from symmetry under spatial translations... But there's no reason why the universe has to have this particular symmetry, it just does.

    On the other hand, knowing what the rules are can be very powerful even if we don't know why they are what they are. Mankind has done some pretty impressive stuff with Newton's laws over the last few centuries.
  5. Oct 25, 2015 #4


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    Reading your post, you may be misunderstanding the term "resistance". In a vacuum, any object (no matter how large) will move as a result of even the smallest force. "Resistance" isn't a "refusal" to move.

    What inertia means is that the more massive an object the less it moves under a given force. Or, more precisely, the accleration is inversely proportional to the mass.

    What would be the alternative? That any object attains the speed of light upon the slightest touch? You just touch a cannonball gently and ... it flies off as though you'd shot it from a cannon?

    On Earth, you have various resisting forces such as friction. So, a cannonball lying on the ground will require a minimum force to make it move at all. If you push it too gently it won't move at all. But, this "refusal" to move is really caused by friction, not directly by inertia.

    In a vacuum, where there is no friction, the cannonball will always move if you push it: its inertia will determine how fast it will accelerate, not whether it will move or not.
  6. Oct 25, 2015 #5
    If you tell me what exactly is a force, I'll tell you what is inertia.

  7. Oct 25, 2015 #6
    What is a force? If you get your brother, sister or a friend to stand next to you put a hand on your shoulder and push what you will feel is a "force". A force is that which changes the state of motion of an object. How great the effect on you ( whether you hit the nearest wall or not) depends on how much "force" the person applied ( how hard they pushed) and your body mass ( that is if we assume your shoes are friction less - generally not true but assume your floor is an air hockey table and the soles of your shoes have been oiled and you should be pretty close) which will determine how much you accelerate. In this case the force could be impulsive - only applied for a relatively short time so your final velocity would be a constant. If on the other hand your partner doesn't have oiled shoes and the floor is not an air hockey table for him, but still is for you, and he continues to push with the same force, you will continue to accelerate and your velocity will continue to increase until he stops pushing or you hit the wall.
  8. Oct 25, 2015 #7
    If you define a force as "what changes the state of motion of a body", then asking what is inertia is meaningless and the problem is solved.
    (I don't intend that your answer is meaningless, only that the answer to the OP question is simple, whatever the definition of force he gives. Your is one, there are others, but whatever it is I can give him a simple answer).

  9. Oct 25, 2015 #8


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    You can have forces in static situations.
  10. Oct 25, 2015 #9
    I don't really like to look at this way. It's valid, but it's not particularly meaningful or instructive.

    First, consider that these two things are the same:
    1. An object not moving.
    2. An object moving in a straight line at a steady speed.

    Any perceived difference is due only to your motion relative to the object. And there's no experiment you can perform that will allow you to determine which of the two things you are doing!

    So in this sense inertia is not a property of the object, but rather a consequence of your (an observer's) motion through space.

    Secondly, when an object does undergo a change in it's velocity, there's a direct proportion between the rate at which its velocity changes (its acceleration) and the force applied to the object. One might wonder why, when the same force is applied to two different objects, they don't have the same acceleration. And the answer is that there's a property these objects have that can be used to explain why this happens. That property is called the mass. But it only explains it insofar as to be able to predict that when an object has a larger mass it undergoes a smaller acceleration. There is nothing more to it than that. It's simply the fact that this property can be used by scientists to make predictions about how things move, and engineers can use it to design and build things that work for us. It's the utility of the concept, not any underlying philosophical consideration, that explains its presence.

    This became quite clear in the very early 1900's when it was shown by both theory and experiment that the direct proportion between force and acceleration is an approximation that's valid only for speeds small compared to the speed of light, and that the energy of its internal constituents make a contribution to the object's mass. After all, how can the validity of a concept have a deeper philosophical meaning when it's no longer valid?

    The closest question to that asked by physicists: What gives the fundamental particles their mass?

    Imagine an astronaut on a space walk using her legs to push off the side of the ISS. Do you suppose that the astronaut and the ISS will undergo the same acceleration? Or do you suppose that the astronaut will undergo the larger acceleration because of her smaller mass?
    Last edited: Oct 25, 2015
  11. Oct 29, 2015 #10
    A simple intuitive answer is that mass has inertia because acceleration imparts kinetic energy to a mass, and it takes work, which is force times distance, to do that. The SI unit of work is the joule, the result of a force of one Newton moving a mass a distance of one meter along the applied direction of force. Naturally, one joule of work done to accelerate a mass increases its kinetic energy by one joule.
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