# Gravity and freefall

## Ideas on Gravity from Newton to Einstein

We have already seen that there is no place in the universe where there is no force. All matter, irrespective of how small, has a gravitational field. Even empty space has a gravitational field acting on it. This also means that there is at least some energy everywhere.

How can empty space transmit force? And how can empty space contain energy?

Einstein became famous when it was proved that light is bent around stars by gravity, as he predicted in his Theory of Relativity.

Before Isaac Newton, people did not understand gravity at all. They thought that things 'went to their natural place', without any concept of how that happened.

But Newton had the brilliant insight to realise that gravity is a force. And this force can act over distance. This means that the Earth and the Moon can exert a pull on each other, even though between the two bodies there is nothing at all. The magnitude of the pull of gravity of the Earth on the Moon is the same as the pull of the Moon on the Earth.

Two and a half centuries later, Albert Einstein furthered our understanding of gravity, with his famous Theories of Special and General Relativity. When his theory was proved correct by Arthur Eddington in 1919, he became world famous, and received the Nobel Prize two years later (though not for Relativity, but his work on the Electromagnetic Effect).

## Force and Medium

Some transfers of energy need a 'medium'. For example, air is a medium for the transmission of sound. That is why films are wrong when they have sound emitting from passing spacecraft and explosions in space.

Gravity is weak! Even a short rubbing of a rod is enough to cause a charge strong enough to overcome gravity.

However, gravity and the other three fundamental forces (electromagnetism, and the strong and weak nuclear forces) do not need anything to travel through. They do not need a medium.

Light can also travel through empty space without a medium. Similarly, radio waves and infrared (heat) do not need a medium. That is why we can talk to astronauts in space by radio, and feel the heat of the Sun. And light is also affected by gravity! A light beam passing a large star will be bent a little due to the gravitational field of the star.

## Gravity is fundamental

Gravity is one of the four fundamental forces of nature. The other three forces are electromagnetism, and two forces found in the nucleus of an atom.

The Moon and the Earth have different masses, so attract objects with different forces. The acceleration due to gravity on the Moon is therefore only 1/6 that of the Earth.

Gravity is produced by mass – any mass. Even between you and your neighbour there is gravitational attraction. But we do not get pulled over every time someone walks past us – this must mean that gravity is very weak. Much weaker than electromagnetism. Even a very weak magnet has no trouble picking up metal objects against the pull of gravity.

Try dropping two objects, such as a pen and a book, from the same height. Did they land on the desk or floor at the same time? So, why do two objects with different masses fall together? Does it mean that the force of gravity is the same for both a light object and a heavy object?

## Gravitational Acceleration

All objects on Earth are accelerated by gravity at the same acceleration – 9.8 m/s2, but the force applied by gravity depends on the mass of the object.

There is gravity on the Moon, but it is only 1/6 (0.17g) that of the earth. Every mass, no matter how small, exerts gravity on all other mass in the universe. But the further apart the masses are, the weaker the pull of gravity between them is. In fact, the force decreases by the square of the distance. If you double the distance there is one-fourth the gravitational attraction.

### Galileo's gravity experiments

Galileo Galilei is reputed to have invented empirical science by carrying out a trial with two objects falling in the Earth's gravity field. This was to test the assertion, which was generally believed, that heavier objects would fall faster than lighter objects.

Galileo Galilei, Italian scientist, astronomer, mathematician, and inventor

Stop for a moment and consider whether this is true, and if not, why not?

It is easy to confuse the effect of air resistance with lower acceleration. A piece of paper reaches terminal velocity, at which the force of air resistance is equal to the force of gravity, causing the paper to float down to the ground at constant velocity. However, scrunch up the paper into a ball, and we observe the same mass falling at a faster rate. This demonstrates that it is air resistance, and not the mass, that causes the difference in the falling rate of the object.

Galileo Galilei ran a series of trials on a sloping ramp to test his idea that all objects will accelerate at the same rate, irrespective of their mass. He also developed an ingenious system, using bells at varying intervals, to measure the acceleration of the balls he rolled. He did this because he had no means of measuring small intervals of time. The story of this clever man is recounted in Vitruvian Boy.

Galilei also posed the question of gravity on the Moon: would a hammer and a feather fall at the same acceleration in a vacuum? In 1971, Astronaut David Scott actually carried out Galileo's thought experiment on the Moon, and vindicated the great Italian genius's insight.

Associated scientists: Newton, Einstein, Galileo, Randall, Lisa Randall, Hawking

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### Great Scientists

#### Albert Einstein

1879 - 1955

Albert Einstein is considered by many to be the greatest scientist of the 20th century, and his contributions to science equal in importance and scope to those of Isaac Newton.

### Quote of the day...

"So," said Sean. "One day the whole world will know about Newton's Laws of Motion?"
"Only if they buy a renewable license," answered Newton. "I write the law. In this world, nothing moves without my say-so."