Put simply, inertia is the measure of an object's reluctance to change its state of motion. The object's state of motion may be stationary or flying through space at 20,000 km/hr. The unit for measuring inertia in the SI is the same as for mass: the Kilogram. Inertia (mass) can be expected to increase when an object is traveling at relativistic (near the speed of light) speeds.

Supervisor's note: The following answer is looking for a clear-headed makeover.

Inertia is NOT a force because Inertia just is the resistance to change in motion. An object's inertia never changes.

Here is something to think about so you can answer the question yourself...

If you slide a pencil on the floor away from any object that could block it the pencil still stops. Do you think it is because gravity or inertia did it? No. The thing is inertia is just like the Law of conservation. All objects,energy, etc. follow it but inertia is not A FORCE that stops a car when it hits a wall. It is an outside force like friction which is what stoped the pencil from countinuing to slide on the ground. An object's inertia can change but not it's amount, because there is no amount of inertia because like i said before inertia is like a law, all objects have inertia (meaning they all follow the rule of inertia) If there were no other outside force like gravity and friction then when we could not accelarate and there would be no inertia. I know this paragraph makes no sence but to me this is how i look at inertia. As a law or property of matter and not a force. One last thing to think about. Does inertia change anything... if so write a disscusion and tell me what you think it changes and if a car crashes DID INERTIA CAUSE THAT...

Inertia cannot send the planets sailing off through the galaxy as long is the sun's gravity is acting on them. Only if the sun's gravity suddenly disappeared would this happen. The pull of the Sun's gravity is the only force keeping the planets from heading off alone through the galaxy. The planets' inertia keeps the Sun's gravity from pulling them into the Sun at the center of the solar system.

It's a nice balance, and it has been going on for about 4 billion years since the solar system formed from the dust and gas scattered through this region of interstellar space.