# The force of inertia

1. Oct 15, 2014

### 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?

2. Oct 15, 2014

### Simon Bridge

3. Oct 15, 2014

### A.T.

4. Oct 22, 2014

### 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).

5. Oct 22, 2014

### 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.

6. Oct 23, 2014

### Nabeshin

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...

7. Oct 23, 2014