The Limits of Kepler’s

Laws

Kepler’s laws allowed the relative size of the solar system to be calculated, but not the actual size.

This could not be calculated until the invention of radar, which could be used to find the distance to Venus.

Adding these two distances, the astronomical unit is defined as 1.5 x 108 km.

Kepler’s laws told how the planets move, but not why.

Isaac Newton (1642-1727) was born in Lincolnshire, England on Christmas Day in 1642, the year Galileo died.

Newton studied at Cambridge, but the arrival of the bubonic plague forced him home for two years. He went to live on his uncle’s farm.

It was there he made one of his most famous discoveries, the Law of Gravity.

For some reason he didn’t tell anyone for 20 years, until he offhandedly mentioned it to Edmund Halley in 1684. Halley encouraged Newton to publish his work, and he did so in a work known as Newton’s Principia.

Newton’s three Laws of Motion, the Law of Gravity, and the calculus are adequate to explain all motion we see here on Earth and throughout the universe.

Newton’s First Law of Motion - objects resist acceleration. (Inertia)

Newton’s Second Law of Motion F = ma

Newton’s Third Law of Motion - To every action there is an equal and opposite reaction.

The Law of Universal Gravitation - Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of the masses and inversely proportional to the square of the distance between them.

Galileo realized the concept of inertia (which conflicted with Aristotle’s belief that the natural state of an object was to remain at rest) long before Newton came up with his first law.

Galileo’s famous inclined plane experiments led him to the logical extension of having no second plane at all.

Newton’s laws describe the way forces affect motion.

Force - a physical quantity that can affect the state of motion of an object. (a “push” or “pull” on an object)

There are two general types of forces: contact forces and field forces.

Forces on an object can be depicted with a force diagram (free-body diagram).

The unit of force is the newton: one newton is the force needed to accelerate a one kilogram mass by one meter per second per second.

NEWTON’S FIRST LAW:

If there is no net force acting on a body, it will continue in its state of rest or will continue moving along a straight line with constant speed.

LAW OF INERTIA