Understanding Inertia to its Uses and Importance in Physics

[bobie]

The force of inertia is the property common to all bodies that remain in their state, either at rest or in motion, unless some external cause is introduced to make them alter this state.Is the concept of inertia still used? When is it useful as a fictitious force?

Can you list a few situations in which, if we didn't use this tool we might be in difficulty?

 

[Simon Bridge]

Is the concept of inertia still used?

Yes and no. Leave off the "the force of" part.
http://en.wikipedia.org/wiki/Inertia

Please provide a citation so we know what the passage you are quoting is talking about.

When is it useful as a fictitious force?

... any time it simplifies the calculation.
For instance, when you are considering motion in a non-inertial reference frame.

Further reading:
http://quod.lib.umich.edu/d/did/did2222.0000.714/--force-of-inertia?rgn=main;view=fulltext

 

[A.T.]

When is it useful as a fictitious force?

In non-inertial frames:
http://en.wikipedia.org/wiki/Fictitious_force

And in quasi static analysis:
http://en.wikipedia.org/wiki/D'Alembert's_principle

 

[Dr.D]

I would disagree with the previous responses and say that it is NEVER useful as a force. Consider these ideas, please:

1. Newton's 2nd law says Sum F = M * A
This means that all of the real forces appear on the left side (or in the first term if you want to set it all to zero). The M * A is not a force, and therefore should not appear in the force sum.

2. There will be some who will say, "well, it looks just like a force when we look at the 2nd law," and that is true. But the 3rd law says that for every action (force) there is an equal and opposite reaction (force) somewhere in the universe. There simply is no reaction to an "M*A" force, so it must not be a force.

Simon noted above that it is useful "anytime it simplifies the calculation." That is hard to argue with except to ask, "how does it simplify the calculation?" The usual effect to cause the user to pay less than full attention to the expression of the acceleration (usually assuming an unduly simple form), so that it leads to an error.

I strongly suggest, leave D'Alembert in his grave and stick with Newton (or Lagange).

 

[Simon Bridge]

Useful or not it is sometimes still used - yes, imagine my shock.
We get a lot of questions here about accelerated reference frames where the calculations are best done in those frames.
... but I prefer not to use the phrase "force of inertia" to help people with them as it's too confusing.

 

[Nabeshin]

2. There will be some who will say, "well, it looks just like a force when we look at the 2nd law," and that is true. But the 3rd law says that for every action (force) there is an equal and opposite reaction (force) somewhere in the universe. There simply is no reaction to an "M*A" force, so it must not be a force.

Well, this is only because such an inertial 'force' does not act upon a body from any external body (except, perhaps a la Newton, from absolute space). I'm not sure the 3rd law is then applicable to it, although this obviously depends heavily on the particular formulation of this law. I'd prefer something like F_{AB} = - F_{BA} rather than the colloquial equal and opposite reaction...

 

[A.T.]

Simon noted above that it is useful "anytime it simplifies the calculation." That is hard to argue with except to ask, "how does it simplify the calculation?"

We already had a thread on this:
https://www.physicsforums.com/threa...ssary-to-solve-certain-problems.537882/page-4