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- Thread starter jnorman
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I thought of it and within classical Physics I don't see the need of the concept inertia at all.

The natural state of an object is to stay at rest or move with a constant speed along a straight line. The law of inertia says mass provides resistance to acceleration, but is this true? This implies the object wants to stay at rest or move at a constant speed along a straight line, but whatever small force I apply to a big mass (in free space) it will accelerate and never stay at rest or move with a constant speed along a straight line. So what I'm saying is the object doesn't resist acceleration at all... it's just as natural for an object to accelerate under a force as it is to stay at rest and move at a constant speed along a straight line without any force applied. Both are natural states.

The law of inertia states mass is a measure of inertia which implies mass is a measure of resistance to acceleration. You could agree with this. It is harder to push an object which has a greater mass then an object with a smaller one. But is this really because the mass provides resistance? I don't think so. I like to use the concept of kinetic energy in this case. Different speeds correspond to different energy levels. There's no resistance there's just more energy needed to reach a higher energy level(which is completely logical). All energy applied is converted into kinetic energy. If you could speak of resistance there surely should have been energy loss. When an object has a greater mass it doesn't have more inertia (in my opinion) it just has more matter to divide it's energy with.

Compare the concept of kinetic energy with filling a cup of water:

Water is the kinetic energy.

The cup is the object.

The surface is the mass.

The change in height is the acceleration.

The height of the water level is the speed.

You could have a cup of water for instance that is empty. Now when I fill the cup could you speak of resistance of the cup (or actually: mass = the surface of the bottom) to the filling? No, the glass just lets the filling happen. There's no way the cup struggles to receiving the water (the object just gets more energy and is gladd to hold this). The surface of the bottom could be bigger which would mean the cup would take longer to fill, but does this resemble resistance or inertia? In my opininion there's just more space to fill.

I would replace the concept of inertia with the following which I think is more correct:

The natural state of an object is -->

a) To stay at rest or move with a constant speed along a straight line if no external force is present.

b) To accelerate if there is an external force working on the object.

Further I would use the concept of kinetic energy and the Impuls-momentum theory.

I just don't see the need of the concept of inertia as it is defined in my textbook: "Inertia is the natural tendency of an object to remain at rest or in motion at a constant speed along a straight line. The mass of an object is a quantitative measure of inertia."

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Let's stick to classical mechanics (despite the fact that this is the quantum mechanics forum). There is a slight problem with the above interpretation of "resistance". Of course, in the sense that an object will accelerate when you force it, it is "yielding" to your influence. But the fact that it *requires* your influence and effort means that the object does *resist* the change in it's velocity.

Thus, the statement of Newton's first law is that an object will be at rest or move with constant velocity in a given inertial reference frame if there are no net external forces acting on it. The fact that, e.g., you have to push the front of a rolling car (ignore the fact that there is already a force provided by the ground/tire rubbing) to stop it from moving means that the car does not want to stop...we then title this resistance to change "inertia".

Then Newton's 2nd law says that we can quantify this relationship via

F(net)=ma (for constant mass).

With the regards for why objects have inertia: In classical mechanics, there is clearly no explanation for why objects have inertia. Its existence is an observation that is then taken as a law.

In the standard model of particle physics (which uses the framework of quantum field theory where mass is a measure of part of the energy content of a particle), mass for fundamental particles can be obtained via particular interactions among quantum fields...massless particles propagating in a non-trivial vacuum can give these particles an apparent mass-energy. There are other mechanisms for the appearance of what we ultimately interpret as mass in theories beyond the standard model of particle physics.

Suffice it to say that the issue of the fundamental nature of inertia has become a technical one, which can no longer be argued based on "first principles" mechanics. Physicists have wrangled a lot of the unexplained postulates of classical mechanics into a fewer number of unexplained postulates in our more fundamental theories.

- #6

nickdanger

Lorentz said:Why is there even a need for the law of inertia? I thought of it and within classical Physics I don't see the need of the concept inertia at all.

I like your idea here. I have a similar problem with the inertia definition. It does not seem usable and that makes me think it is incomplete. I think that the problem starts with the classical thinking that all force applied to a body goes into kinetic energy. (Of course, some converts to a mass equivalent according to relativity, but this is a small amount.). How do we know that some of the energy is not going into some other type of energy, let's say just its "inertial content". Like balancing thermodynamic energies of various types this may be a reservoir of "formation energy" that has to do with the energy of the mass that holds the object together as one mass. So we should think of it as the force of acceleration is being split between the two energy reservoirs: Kinetic and formation, and so wonder what the rule of this conservation is.

I say this because we look at Galileo's conclusion from the ramp experiment as the inductive leap that started the inertia idea. His conclusion was that the ball rolling down a ramp onto a flat surface must continue on forever at the same velocity if all opposing forces were removed. Classical energy theorem analysis agrees. But why should we believe the ball will go forever. Only a finite amount of energy was tranferred to the ball. Why would that transfer a constant mass (due to relativity - the small amount) and also a kinetic energy over all time without ever being spent. How do we know the ball won't start slowing down eventually? We believe it will only receive a finite velocity with a given force, why then should it have that finite velocity for infinite time? Wouldn't the energy that provided the impulse radiate away slightly over time?

Scientists are looking at the Pioneer 10 mission and saying that the probe started slowing when it went past jupiter (Discover, Oct 2003). It was getting further from any gravitational pull, especially from the sun. It should have been at a final constant speed and there was no gravitational source to slow it. So scientists are looking at dark energy that is being used to explain the same problem with expanding galaxies. However some are looking to modified newtonian dynamics (MOND) and saying the Newton's standard law is not valid at extremes of "low acceleration". But I think it may be that we have to rethink the inertia idea.

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