Inertia is the resistance of a mass to change in its state of motion. A heavier mass requires a larger force to move it from rest, as represented by the red vector. Once in motion, an equivalent force in the opposite direction is needed to stop the object. In the absence of such a force the mass will continue to move with constant velocity, according to Newton's first law of motion. This Demonstration depicts a simple experiment, in which a small car with an object on top hits an obstacle, and the object continues its motion in the same direction and with the same speed.