# Question on Inertia [closed]

I'm trying to get the concept right.

If an object is accelerating in a straight line, with no opposing forces acting on it, will it continue to accelerate forever due to its inertia?

## closed as unclear what you're asking by Yashas, David Hammen, Jon Custer, honeste_vivere, John RennieJul 18 '17 at 5:16

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• I think you are mixing acceleration and velocity. You should start figuring this out from a text book or wikipedia articles. – Communisty Jul 17 '17 at 12:02

The rule that ties forces to motion is Newton's 2nd law:

$$\sum F=ma$$

It shows the answer right away. When there are forces, there is acceleration. No forces, $\sum F=0$, means no acceleration, $a=0$.

And no acceleration is another way of saying no change in speed (velocity). So, a drifting spaceship will forever drift. A speed will forever stay the same, until forces appear to change it (by causing acceleration).

There are no rules or laws like this that tie velocity to forces. Such have never been found. Forces and velocity are simply not related. You do not need one to have the other.

• There are no rules or laws like this that tie velocity to forces. Such have never been found. Forces and velocity are simply not related. You do not need one to have the other. Ideally, I agree. But practically, being pedantic, (possibly), to get to a given velocity, I can't think of a single real physical object that ALWAYS had a given constant velocity. Acceleration eventually gets involved to give a $\Delta V$. Of course, it takes only one example to falsify this, I just can't think what that is in the real physical universe. There is always a potential somewhere? – user163104 Jul 17 '17 at 14:21
• @JohnKennedy You are of course right that in real life there is very often some change of velocity going on - and why? Because there is very often som net force going on. There is a law tying force to acceleration but not one tying force to velocity. (By the way, a satellite is not so far off from having constant speed...) – Steeven Jul 17 '17 at 15:31

If there are no forces acting on the object, then it's not accelerating ─ it's moving at constant velocity in a straight line and with zero acceleration. In this case, if the forces don't change, Newton's First Law tells you that the situation will continue indefinitely.

If the object is accelerating, then that means that there is a force acting on it. If the force continues indefinitely, then the acceleration will continue indefinitely. If the force gives out, for whatever reason, then the acceleration will stop.

As long as there is a net force applied then it will accelerate.

More generally, a net force increases momentum over time. Inertia is the scaling factor between velocity and momentum (i.e. for the linear case inertia = mass). Think of inertia in term of momentum and not forces

$$\mbox{(momentum)} = \mbox{(inertia)} \times \mbox{(velocity)}$$

and

$$\mbox{(forces)} = \frac{\mbox{(change in momentum)}}{\mbox{(time step)}}$$

The above applies for both linear and rotational motion. Inertia does not cause the acceleration in your scenario, it is merely fixed scaling factor describing how much acceleration a body has.