Re: Is inertia a force? Why or why not?

If you use our search engine (http://www.madsci.org/MS_search.html) and search on "what is inertia" with "find exact phrase" checked, you will find a previous answer which describes what inertia is, and I suspect you have recently discussed this in your physical science or physics class. You know, then, that inertia is the tendency of an object to remain in whatever state of motion it presently exists, and that it takes a force to change it's state of motion. But inertia is not itself a force.

You also already know, I'm sure, that we can calculate the amount of force needed to change the motion of an object, by the equation
f = m a
where f is force, m is mass, and a is acceleration. Since acceleration is the time rate of change of velocity, we can replace "a" with "Dv/Dt" where Dv is the change (or "delta") of velocity and Dt is the change ("delta") of time. So we have
f = m Dv/Dt
Now, m Dv is just the change in momentum, which we can write as Dp (most physicists use "p" for momentum), and so we have
f = Dp/Dt
which describes the fact that the magnitude of a force can be "measured" by determining the time rate of change of momentum. Physicists often use momentum as the measure of an object's inertia, so we can say that a certain force is required to change the inertia of an object in a certain amount of time (assuming the quantities are constant). This formula, by the way, is the form that Sir Isaac Newton used for his famous "f = m a" equation.

I hope this helps you!

John Link, MadSci Physicist