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• Gravity is a force that attracts all objects towards each other. People are attracted towards the Earth and the Earth towards people, the Moon and the Earth are attracted towards each other, and the Sun and the Earth are attracted towards each other. All of these attractions are caused by gravity. Gravitational attraction is greater for more massive objects. Gravity decreases as distance between the objects increases.

Rights: University of Waikato

Gravity keeps satellites in orbit

Dr Allan McInnes and Dr Adrian McDonald, from the University of Canterbury, explain how gravity and projectile motion keep natural and artificial satellites in an orbital path.

## Gravity attracts all things towards each other

Every object in the Universe is being attracted towards every other object by the force of gravity. This means that there is nowhere you can go in the Universe where gravity is not acting. Examples of gravity in action:

• Gravity holds the atmosphere in place around the Earth.
• Gravity keeps people on the Earth’s surface.
• Gravity keeps the International Space Station in orbit around the Earth.
• Gravity keeps the Moon orbiting around the Earth.
• Gravity keeps the Earth orbiting around the Sun.

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

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.

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.

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 1,000 kg has a weight force of 9,800 N at the Earth’s surface. The radius of the Earth is about 6,366 km, so at 6,366 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 2,450 N.

So why doesn’t a 1,000 kg satellite just fall back to Earth?

### Sideways speed keeps satellites in orbit

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.

Rights: The University of Waikato Te Whare Wānanga o Waikato

Satellite speed and force directions

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.

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

## Nature of science

Science ideas change over time. Newton’s gravity-based world view has since been superseded by 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.

## Related content

Since the earliest times, humans have made observations of the night sky. These observations led to the development of models to explain the movement of these natural satellites. Explore this further in Our solar system – revolutionary ideas.

In the recorded Physics made simple – gravity webinar we use simple examples and take you step by step through how you can teach the gravity concept and also unpack students’ alternative conceptions.

The article The gravity well – a physics analogy explains how to construct and use the gravity well demonstrated in the webinar. It includes an interactive with conceptual simulations that can be demonstrated on a gravity well and links to related Hub content.

## Activity ideas

Beginning a teaching sequence with Gravity and satellites – true or false? The activity is supported by the teacher resource Alternative conceptions about gravity and is a useful pretest to establish student understanding.

Then get your students to turn their eyes to the night sky to observe natural satellites and to spot artificial satellites – like the ISS – as they pass overhead. Back indoors, students can hunt for satellites online with a webquest.

Scale model for satellite orbits explores satellite altitudes and satellite motion and uses models to teach science concepts.