2.1 Newtons First Law of Motion
Newtons first law of motion states that an object with uniform velocity will continue to maintain that uniform velocity, unless acted upon by an outside force. This law is known as the principle of inertia. Inertia is the tendency of an object in motion (or at rest) to stay in motion (or at rest). Mass, or the amount of matter (the substance from which all physical objects are made) is the measure of inertia. The greater an objects mass, the greater its inertia. This concept is easily demonstrated in real life: the more massive an object, the harder it is to accelerate. It is much easier to stop a plastic toy car traveling at 10 m/s than it is to stop a large truck traveling at the same velocity. Likewise, kicking a soccer ball into motion is not as difficult doing so to the same size cannonball.
What is the relationship between mass and forces? Notice the last part of the first law: "unless acted upon by an outside force." Forces are related to changing motion. Newtons second law of motion furthers this idea.