Newton's work in mechanics can be summarised in a statement of his three laws. The first tells us what happens in the case of a body on which the net force is zero.
Every body continues in a state of rest or of uniform velocity unless acted on by an external force.
The acceleration of a body under the action of a net force is directly proportional to that force and inversely proportional to the mass of the body. To every action there is an equal and opposite reaction. The third law will not have much impact on the progress of this course, so we will not consider it further. To give you an idea of the application of this new understanding of mechanics, let us look at the old question of whether, when two bodies are dropped from the same height, the heavier would reach the ground first, a question answered in the affirmative by Aristotle. Let us consider the case of an elephant falling out of a tree.