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Forces

Gravity

Rollercoasters are pulled downwards by gravity.Rollercoasters are pulled downwards by gravity. The Universe is made of matter. Matter is held together and moved by forces. One of the basic or fundamental forces is the gravitational force—also more simply known as gravity. Any piece of matter from a pinhead to a planet has gravity. It pulls or attracts other matter. The biggest large lump of matter in our daily lives is the Earth. Every time you jump up in the air, you fall back down to Earth again. The Earth's gravity pulls us and other objects towards it, keeping our feet on the ground.

Isaac Newton observes an apple falling to the ground.Isaac Newton observes an apple falling to the ground.

Newton

The the English scientist Sir Isaac Newton (1642–1727) was the first person to understand that gravity affects objects in space, like the Moon, Earth and Sun, in the same way that it affects things on Earth—like us. The story goes that he was struck by this idea when he saw an apple fall to the ground. Both the apple and the Earth have gravity, but because the Earth is huge compared to the apple, we see the apple moving towards the Earth and not the other way round.

Newton also realised that the nearer an object is, the stronger is its force of gravity on other objects. The further away it is, the weaker the force. The Earth is very big and very near, so for us its gravity is strong. However, as you travel further away its gravity weakens, although even at the distance of the Moon, it is still strong enough to keep the Moon in orbit around the Earth.



The planets are held in orbit by the Sun's gravity.The planets are held in orbit by the Sun's gravity.

The Sun’s gravity

The area over which an object’s gravity extends is called its gravitational field. The largest object by far in our Solar System is the Sun. It has a huge gravitational field which pulls all the planets—some hundreds of millions of kilometres away—towards it. However, they remain in their orbits and never collide with the Sun because they are travelling fast enough for their momentum (the tendency for moving objects to carry on moving) to balance exactly the Sun’s gravitational force. In space there is no air resistance to slow the planets down, so they carry on moving at the same speed.




An astronaut standing on the Moon's surfaceAn astronaut standing on the Moon's surface

Mass and weight

The strength of gravity depends on an object's mass: the amount of matter it contains and how densely packed its atoms are. A tennis ball has less mass than a cannon ball. The Moon is less massive than Earth, so its gravity is less. The Sun is more massive than Earth, so its gravity is greater.

Weight is different from mass. Weight is a measure of how much gravity is acting on an object. A big book is weighty because it is being pulled downwards by Earth’s gravity and we have to counteract this force with our muscles when we pick up the book. Weight varies according to the strength of gravity. On the Moon, where gravity is less than on the Earth, the book would weigh only about one sixth of what it weighs on Earth. On a star consisting of vast amounts of matter—giving it extremely high gravity—the book would weigh many tons. The book's mass, however, remains the same wherever it is.




Weightlessness

Weightlessness is something you feel when there is no object either pushing or pulling you. When you sit on a seat you experience two forces: the force of the Earth's gravity pulling you downwards and the force of the seat pushing you upwards, known as the contact force. When you are lifted out of your seat on a rollercoaster, you feel weightless for a brief moment—until gravity pulls you back on to your seat.

But if your seat wasn't there, your feeling of weightless would continue, because there is no contact force (previously provided by your seat) balancing the pull of gravity. You would be in free fall, hurtling towards the ground.

Weightlessness aboard the International Space StationWeightlessness aboard the International Space Station Astronauts on the International Space Station feel weightless because they are in free fall all the time. Gravity still has a strong pull on the Station—it is only about 350 kilometres (218 miles) above the Earth—but because it orbits the Earth at high speed, its momentum balances the Earth's gravity. This is the same as what happens with the planets orbiting the Sun. The Station stays at the same height above the Earth, along with the astronauts safely inside it.




Demonstrating the laws of gravity on the MoonDemonstrating the laws of gravity on the MoonClick to play video

Acceleration

Objects falling to the ground move faster and faster: they accelerate. This is due to gravity. The Italian scientist Galileo showed that all objects fell to the ground at the same rate, however great their mass. In practice an object cannot fall freely because of air resistance acting against gravity. A feather and a hammer have different mass, but a feather would fall to the ground on Earth more slowly because it would be held up by the air. This would not be the case on the Moon, however, where there is no air.


Consultant:
 Dave Hawksett

Gravity is one of the four fundamental forces of nature, along with electromagnetism, the nuclear strong force and the nuclear weak force.

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