Isaac Newton (1642-1727)

Isaac Newton was one of the greatest scientists of all time.

The laws he developed revolutionized the way we think about the universe once again.

Born a century after Copernicus published his book, Newton’s work build on and greatly extends the work of Galileo and Kepler.

The main points of Newton’s work are summarized in his three laws of motion and his law of universal gravitation.

Newton’s laws of Motion

1.An object tends to remain in motion at the same speed in the same direction. (Law of Inertia)

Examples: a rolling ball, a hockey puck.

2. The acceleration of an object is proportional to the net force acting on it and inversely proportional to the mass of the object: a = F/m (or F = ma.)

-acceleration is defined to be the change in speed or direction of an object.

Example: pushing a truck/car, rock on a string

3.If object #1 exerts a force on object #2, object #1 feels a force equal in magnitude but opposite in direction to the force it is exerting.

Examples: rowing a boat, jumping, throwing, walking.

Newton’s 3 laws of motion constitute a simple, powerful theory of motion of all objects.

Newton’s law of Universal Gravitation

Newton identified that the same force that held everyday, terrestrial objects to the Earth was the same force that held the moon in orbit around the Earth and the planets in orbit around the Sun. His law of gravity states:

There exists an attractive force between any two objects that has a magnitude proportional to the product of the masses of the two objects and inversely proportional to the square of the distance between the centers of the two objects.

Note that this law nor the laws of motion distinguish between everyday objects and celestial objects (like Kepler’s laws of planetary motion). Everything in the Universe is subject to the same laws. (Thus the word "Universal" in the title of the law)

Planetary Motion

Newton’s laws allow us to explain planetary motion in the same ways we describe any other type of motion.

A planet traveling around the sun (or moon traveling around a planet) is much like twirling a rock on a string.

The force on the rock is due to the string, the string pulls the rock toward the center.

The force on a planet is due to the gravitational force that pulls the planet toward the center.

If the orbiting moon is moving fast enough the Gravitational force can’t pull it fast enough to crash it into the surface of the planet. So the moon continues to fall, always "missing" the surface of the planet. (See fig. 5.3)

Thus, objects in orbit are continually “falling’ around the Earth.

Why are astronauts in orbit “weightless”?

Newton’s laws and Kepler’s laws

Newton was able to show that the laws Kepler had found could be explained using his laws of motion and the law of universal gravitation.

Newton’s laws apply to all objects (including planets) where Kepler’s laws apply only to planetary motion.

Thus Newton’s laws are a model for describing the motion of everything in the universe. They don’t prove that Kepler was wrong, but instead show they are consequences of a much broader, more inclusive model of the Universe.

Note that with Newton’s laws Kepler’s 3^rd law of planetary motion becomes:

For a planet and the Sun the planet’s mass is negligible.

Other consequences of Newton’s laws.

Tides – The moon exerts a greater gravitational pull on one side of the Earth than on the other. So it stretches the Earth causing the ocean levels to change. As the Earth spins different sides of the Earth are closer to the moon so the tides move around the planet.

Precession – The moon’s gravitational pull also causes the Earth’s axis to change direction, much like a spinning top. So that in several thousand years Polaris will no longer be near the North Celestial Pole.

Highly accurate predictions of planetary orbits – The planets attract each other gravitationally as well, so the orbits of the planets can be even more accurately calculated. (This will have major consequences when we talk about the discovery of the planet Neptune.)

Prediction of periodic comets- Comets could be explained as very small objects in orbit around the sun with highly elliptical orbits. (Fig. 2.10) Edmund Halley was the first to predict the return of a comet.

Newton and Einstein

In much the same way that Newton showed that Kepler’s laws were consequences of his universal laws, Einstein was able to show that Newton’s laws of motion and gravitation, were consequences of even more models known as the special and general theories of relativity.

Current day astronomers and physicists (like Stephen Hawking) now call Einstein’s theories "classical". This implies that new theories (like string theory) can or will explain Einstein’s work as consequences of the new model.

Science never finishes. We keep working to find better and better models of our universe. All these models continue to be testable and make predictions and therefore are never "proven."

Was Newton Right?

Was Kepler Right?

Was Copernicus Right?

Was Ptolmey Right?