Gravity work

Isaac Newton was the first to come up with the idea that all objects are attracted towards each other by
gravity. Even people are attracted towards each other by gravity, but this force is so small that it is
not noticeable. Gravity only becomes noticeable if one (or both) of the objects has a lot of mass, such as
the Earth.

There is gravity in space

Earth-Moon system and gravity

Any two masses are attracted towards each other by gravity. This force of gravity causes the
Moon to change direction to make it orbit around the Earth.

Gravity doesn’t disappear just because you are above the atmosphere. Even if an object is high
above the Earth’s atmosphere, there will still be a strong force of gravity pulling it towards the centre of the


Earth. At an altitude of 30 km, you would be above 99% of the Earth’s atmosphere. At 100 km, you
would
officially be in space, yet the weight force of gravity would still be nearly the same. You and the
Earth
would still be pulled together.

Isaac Newton worked out that, if the distance from the centre of the Earth doubles, gravity becomes
a quarter as much as it was on the surface. A satellite with a mass of 1000 kg ha
s a weight force of 9800 N at the Earth’s surface. The radius of the Earth is about
6366 km, so at 6366 km above the Earth’s surface, the distance from the centre of the Earth will have

doubled. The weight force pulling it towards the centre of the Earth will now only be a quarter as much
but will still be 2450 N.

So why doesn’t a 1000 kg satellite just fall back to Earth?

Sideways speed keeps satellites in orbit.

As a satellite moves around the Earth in a circular orbit, the direction of the force of gravity is
always towards the centre of the Earth.

At an altitude of 100 km, you would be so high that you would see black sky and stars if you looked
upwards. If you took a satellite to this height and released it, it would still fall towards the Earth because
the force of gravity is nearly the same as it is at the Earth’s surface.

However, if the satellite is given speed in any direction horizontal to the surface of the Earth, it
will travel further before it hits the Earth. If it is given enough speed, it will travel so far that,
as it curves towards the Earth, it will miss the Earth altogether. At just the right speed, it will move
around the Earth in a circular motion. This type of motion and the path that a satellite moves in is
called an orbit.

Close to the Earth at an altitude of 100 km, a satellite needs to be moving at 8 kilometres
per second (28 000 km/h) to stay in orbit. At higher altitudes, satellites do not need to be
travelling as fast. Television communication satellites are at a higher altitude of 36 000 km and
only need to travel at 3 km/s (11,000 km/h).

The Moon is 360,000 km from the Earth and only needs to be travelling at 1 km/s to stay in orbit around
the Earth.

If there is gravity in space, why do astronauts appear weightless?

Astronauts appear to be weightless for the same reason that a person on a trampoline feels weightless
when in the air. There is still the same amount of gravity acting, but there is no floor pushing upwards o
n the astronaut, so the weight force cannot be felt.

If a person was in an elevator and the cables broke and the brakes failed (we are assured this cannot
happen), the person and the elevator would fall towards the Earth at the same rate. The floor would
not be holding the person upwards, so the person could enjoy the sensation

n of weightlessness (for a brief while).

This is the same for astronauts high above the atmosphere on the International
Space Station (ISS) at an altitude of about 400 km. Gravity is still strong, but the
astronaut and the ISS fall towards the ground at the same rate. They are also

both travelling horizontally at 28,000 km/h. As they fall towards the ground, they travel
so fast horizonta
lly

that they miss the Earth altogether and orbit the Earth once every 90 minutes.

Nature of science

Science ideas change over time. Isaac Newton’s gravity-based world view has since been
superseded by Albert Einstein’s ideas that all masses distort space and time. This highlights the
fact that science is not a fixed body of knowledge. Although Einstein’s theory is widely accepted,
Newton’s law of universal gravitation is still used for practical situations such as satellite motion.

Main ideas

Gravity is everywhere. Every object has a gravitational pull. Isaac Newton found out how gravity
works because a apple fell from the apple tree and hit him in the head, he thought why did the apple
fall down but not up.There is also no gravity when your floating through space unless you land on a
planet  or a moon.