Exploring Our Fluid Earth

SF Fig. 6.9. Sir Isaac Newton

Image by Sir Godfrey Kneller (1689)

Sir Isaac Newton (1643–1727) was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian. He is famous for describing many scientific principles and credited with co-inventing calculus and the reflecting telescope (SF Fig. 6.9).

In his monograph Philosophiæ Naturalis Principia Mathematica (1687), Newton defined a set of physical laws of motion of objects when forces are applied. This work is the basis for what is now know as Classical Mechanics, and is still being used by scientists today.

Newton’s First Law states that an object at rest will stay at rest, and an object in motion will stay in motion, unless acted upon by a force. For example, if you are a passenger in a car without a seatbelt and the driver suddenly slows down, your body will have a tendency to continue on its original path with the same velocity and you will feel as if you are being pushed forward. If you were wearing a seatbelt, the seatbelt would apply a force to your body, preventing you from continuing on your original path with the same velocity and, instead, keeping you in your seat. This property of objects to resist changes in motion is known as inertia.

Newton’s Second Law states that the acceleration of an object in the direction of the net force acting on the object is directly proportional to the net force and inversely proportional to its mass. We can see this demonstrated in the equation F=ma where F (or force) is equal to mass times acceleration. Thus, in order to accelerate a large heavy object, like a sailboat, across a lake you need to apply a large force, in this case the wind. The standard (SI) unit of force is the Newton, named in Sir Isaac Newton’s honor.

Newton’s Third Law states that for every action there is an equal and opposite reaction. Consider two swimmers, each floating in an inner tube on the water. If both swimmers push off each other at the same time, they will float away from each other in opposite directions.

Newton is popularly remembered for observing an apple falling from a tree and then realizing that a gravitational force must exist between the apple and the earth. Newton defined the Law of Universal Gravitation, which states that any two objects in the universe are attracted to each other with a force that is directly proportional to their respective masses and inversely proportional to the square of the distance between them. There was an attractive force between the apple that Newton observed and the earth. We call this force gravity. Newton noted that this gravitational force would apply to larger bodies such as planets and moons as well.

Newton used the Law of Universal Gravitation and the Laws of Motion to explain orbits of planetary bodies. His work supported previous scientists’ suggestions of a heliocentric (sun-centered) solar system and provided the theoretical framework for the idea that the universe was not geocentric (earth-centered).