According to Newton's First Law of Motion, an object at rest tends to stay at rest and an object in motion tends to stay in motion with the same velocity and in the same direction unless acted upon by an unbalanced force. An object like a car will tend to either keep standing still or keep moving in a straight line at the same speed unless some unbalanced force makes it do otherwise. Examples of unbalanced forces are air resistance, friction between tires and the road, and a push from a tow truck.

Newton's first law is sometimes referred to as the "the law of inertia." Inertia means an object's tendency to resist changes in motion. Mass and inertia are directly related. A school bus, which has more mass than a car, has a correspondingly greater resistance to getting going, braking and turning corners.

When you start a car's engine, fuel is mixed with air, fed into the cylinders and ignited. The mixture explodes. Expanding gases push the pistons down, which turns the crankshaft. When you shift the transmission into gear, power is transmitted from the crankshaft to the wheels and the car starts to move. The forces acting upon the car have become unbalanced, and the car begins to accelerate.