Galileo further observed that regardless of the angle at which the planes were oriented, the final height was almost always equal to the initial height. If the slope of the opposite incline was reduced, then the ball would roll a further distance in order to reach that original height. Isaac Newton built on Galileo's thoughts about motion. Newton's first law of motion declares that a force is not needed to keep an object in motion. Slide a book across a table and watch it slide to a rest position.
All objects resist changes in their state of motion.
All objects have this tendency - they have inertia. The tendency of an object to resist changes in its state of motion is dependent upon mass. Inertia is that quantity which is solely dependent upon mass. The more mass which an object has, the more inertia it has - the more tendency it has to resist changes in its state of motion. Suppose that there are two seemingly identical bricks at rest on the physics lecture table.