In most textbooks the law of inertia is stated as follows ... an object at rest tends to stay at rest, an object in motion maintains its state of motion unless acted upon by an outside force.

What does this mean?

It means that if an oject is sitting still it will remain sitting still unless something pushes it. It also means that objects that are moving will maintain the same speed and direction unless something pushes on them to slow them down, speed them up, or make them change direction. You may ask why your car stops when you are driving on a flat road when you take your foot off the pedal. The answer to that is friction provides a stopping force. If there was no friction, your car would continue to move on that flat road at the same speed and direction when you took your foot off the pedal.

Do all objects have the same amount of inertia?

The answer is no. Inertia is measured by mass. The larger the mass of an object the larger the inertia. This should make sense to you. The larger an object is the harder it is to stop it once it is moving. The larger an object is the harder it is to move it when it is stopped.

Examples