Unit 2 : Activity 2 : Ptolemaic Model : Copernican Model : Brahe : Kepler : Galileo : Newton : Unit Exam


It was Sir Isaac Newton who was able to show that Kepler's laws of planetary motion are a natural consequence of simpler and more general descriptions of motions in nature. This brought into one theory both our observations of how things move on Earth and how the planets move in the heavens. These motions are described formally as Newton's laws of motion and gravity.

Isaac Newton was born in 1642, the same year that Galileo died.

Newton’s First Law

Every mass, or body, be it at rest or moving, will not alter its present state unless acted upon by an external force.

This law is also known as the law of inertia. In less formal language: You can't start to move, slow down, stop moving, or speed up unless there is some force. Galileo was the first to point out this principle. Newton once said he "stood on the shoulders of giants" and Galileo must have been one of those giants!

Newton’s Second Law

The change in direction or speed of an object, is proportional to the amount of force acting on that body. The change is motion is also in the direction in which the force is acting.

Less formally: If I push something, it will move in the direction I am pushing. If something is already moving and I push perpendicular to its motion, it will change the direction it is moving.

The mathematical form of this law is:

where ‘F’ is the force measured in Newtons, ‘m’ is the mass measured in kilograms, and ‘a’ is the acceleration of the mass measured in meters per squared second (m/s2). F and a are vectors, which for now means just that we need information about direction as well as how much force or acceleration. By the way, scientists use "acceleration" to describe both speeding up and slowing down - anything that changes your speed.

Newton’s Third Law

For every action there is an equal and opposite reaction.

That means: If I throw a ball, so I push it in a particular direction, it will push me in the opposite direction. This is very familiar to anyone who has fired a rifle, as it will push quite hard against your shoulder! This law is the basis for jet engines and space rockets, so we'll be talking about it more later.


Before we can relate the orbits of the planets to things falling out of trees on Earth, we need to know the distances in the solar system in the same units (feet, meters, kilometers) that we use on Earth. This turned out to be quite a challenge, and that is the story to be told in the next unit.

Congratulations! You have completed the second unit of Evening Star. Once you feel comfortable with the material, please proceed to the end of unit quiz.