Hello, my name's Dr.

George, and this lesson is called, 'Orbital Motion of Artificial Satellites.

' It's part of the unit: 'Gravity in Space.

' Here's the outcome of the lesson.

I can describe changes to a satellite's speed and velocity as it orbits Earth.

And here are the key words.

I'll introduce them as we go along, but you can come back to the slide anytime if you want to check the meanings.

The lesson has two parts.

They're called satellites and speed of orbit.

Any object in orbit around a planet is a satellite of that planet.

There are natural satellites such as the moon, which is Earth's only natural satellite.

It travels around Earth in a circular path at a distance of about 38,000 kilometres from Earth.

And this orbit takes about 27 days to complete.

Earth also has many artificial satellites which have been launched into orbit on large rockets since 1957.

The picture shows a model of Sputnik 1, which is the first artificial satellite.

The spherical part is about 60 centimetres across.

Currently, Earth has over 12,000 satellites in various orbits doing a range of different jobs.

These include communications, for example, enabling us to use mobile phones, television, weather monitoring and spying.

Now here is a question for you.

Io is one of the large moons of the planet Jupiter.

Which of these statements describes Io? And when I ask you a question, I'll wait five seconds, but if you need longer, just press pause and press play when you have your answer ready.

And the correct answer is B, Io is a natural satellite of Jupiter.

It's not counted as a planet because it's not orbiting directly around the sun.

Many artificial satellites are launched into what's called low Earth orbit.

These have orbits between 300 and 2,000 kilometres above the Earth's surface, which is relatively low for a satellite.

Because these satellites are relatively close to Earth's surface, microwave signals which travel at the speed of light can be sent to and from them very quickly.

And that's how we communicate with the satellites.

This type of satellite is often used in observation systems, observing what's happening on the Earth's surface and creating images.

And which of the following heights would a satellite be in a low Earth orbit? Since low Earth orbits are between 300 and 2,000 kilometres, the answer is B and C.

Only a few satellites such the International Space Station, ISS have a crew, have people on them.

ISS was launched in sections between 1998 and 2011, and it's been occupied since 2000.

The crew carry out a wide range of experiments and they test technology for future missions to the moon or to other planets.

The ISS orbits at a height of 400 kilometres above the Earth's surface and completes 15 orbits per day.

It's large enough to see from the ground with the naked eye.

In fact, it's the third brightest object in the sky after the sun and moon.

It's bright because it reflects sunlight just as the moon does.

It looks like a bright fast moving star visible for just a few minutes at a time close to sunset or sunrise.

NASA has a website called Spot the Station where you can look up a location near you to find out when the ISS will be visible.

Some satellites are in polar orbits.

They travel around Earth going over the north and south poles repeatedly.

These satellites orbit between 500 and 800 kilometres above the Earth's surface and they complete 10 to 15 orbits per day depending on their exact heights.

As the satellites travel, the Earth rotates beneath them, so they pass directly above most parts of its surface each day.

Weather satellites are usually in this type of orbit as it allows them to take detailed pictures each day of much of the Earth's atmosphere.

The picture shows a hurricane photographed by a weather satellite.

Now, which of the following statements about satellites in a polar orbit are correct? The correct answers are C and D.

Some of them are weather satellites and they're outside of Earth's atmosphere, they're above it.

They don't travel directly above the equator.

They're not going around the Earth that way.

They're going between the two poles and they don't pass above exactly the same ground in every orbit.

Because as they swing between the north and south pole, the Earth turns below them.

There's another kind of orbit called a geostationary orbit.

Satellites in geostationary orbit are in a ring 38,000 kilometres directly above the equator.

They travel at a speed that keeps them directly above a fixed point on the equator.

And that means that they appear stationary in the sky to anyone looking up from Earth.

Television broadcast satellites are placed in geostationary orbit so that a satellite dish for picking up the TV signal doesn't have to move to receive the signal from the satellite.

It can just constantly point in the same direction, and that's where the satellite will always be.

Geostationary satellites orbit at the same rate that Earth rotates.

So one complete orbit takes exactly 24 hours.

Here are some images showing Earth viewed from far above the North Pole and a geostationary satellite.

The key shows what each symbol means.

If this is a starting position, if we are at the point on the equator where the red triangle is and we look up, we'll see the satellite directly above.

Six hours later is a quarter of a day, a quarter of 24 hours and so the Earth has done a quarter of a turn and the satellite has moved a quarter of the way around its orbit.

Again, if we look up from where the red triangle is, we'll see the satellite directly overhead.

And 12 hours after the start, the Earth has done a half turn, the satellite has done half of its orbit, and it's still directly above from the point of view of the person at the red triangle.

You may have noticed that the word geostationary includes geo, meaning Earth, as also found in the words geography and geology and stationary, meaning not moving, relative to the ground.

New geostationary satellites have to be assigned particular slots so they don't crash into other satellites that are following the same path.

Because they're so high above the Earth, three geostationary satellites if spaced correctly can between them see almost all of the Earth at once.

How long does it take a geostationary satellite to complete one orbit of Earth? And the answer of course is one day, 24 hours.

And now here's a couple of written questions for you.

Can you explain the difference between Earth's natural satellite and an artificial satellite in low Earth orbit? And can you also explain why a geostationary satellite appears to be stationary in the sky? Press pause when you write your answers and press play when you're finished and I'll show you some example answers.

So here are some example answers.

Earth's natural satellite is the moon.

It's a naturally occurring huge ball of rock, which is about 28,000 kilometres away from Earth, orbiting every 27 days.

An artificial low Earth orbit satellite is launched from Earth's surface by a rocket and placed in orbit somewhere between 300 kilometres and 2000 kilometres above the Earth's surface.

It takes between one and three hours to orbit depending on its height.

A geostationary satellite is an orbit directly above the equator, and it completes one orbit in exactly one day.

This means that as it orbits, Earth rotates beneath it at the same rate.

It's always above the same place on the ground.

Compare your answers and check whether you made some of the same points and well done if you did.

Now let's move on to look at speed of orbit.

Most satellites travel around Earth in circular orbits.

These satellites are therefore in circular motion.

As they travel, their speed remains constant, but they change direction of travel all the time so their velocity is always changing.

Because velocity is a vector.

It includes the direction as well as the size of the speed.

The change in velocity is an acceleration.

And for something to accelerate, there needs to be a resultant force acting on it.

That force is Earth's gravitational pull on the satellite.

That's what makes it accelerate.

So at the four points shown here, the movement direction, the direction of the velocity is as shown by these arrows.

And the gravitational force is as shown by the solid arrows always towards the centre of the Earth.

Satellites can travel at a constant speed in orbit because the gravitational force is the only force acting on them.

And it acts towards the centre of Earth, so it's always at right angles to the direction of movement.

When the force is at right angles to the movement, then only the direction changes, not the speed.

There are no frictional forces in space as long as the satellite is above the atmosphere.

And so a satellite will not slow down.

It doesn't need a thrust force to keep it moving, just as the moon doesn't need a thrust force to keep it going around the Earth.

Which force acts on a satellite in geostationary orbit? It's the gravitational force.

The forces acting on an object following a circular path, like an orbit, can be modelled by a simple experiment.

And you're going to do this after I've explained it.

A rubber bung has a strong thread attached to it, and that thread is passed through a thin plastic tube as shown here.

The other end of the thread has masses hanging from it.

You hold onto the tube and you can spin the bung by moving the tube and you can make it follow a circular horizontal path.

So this is a view from above.

And in this model, the bung represents an orbiting satellite.

The direction of its movement is constantly changing.

It's moving in a circle.

Here's its movement direction at these four places.

And the force towards the centre is provided by the tension in the thread, and that represents the gravitational force that acts on the satellite towards the centre.

The size of the tension in the string can be buried by altering the size of the mass hanging from it.

Increasing the mass increases the tension, and this simulates increasing the size of the gravitational force.

So now in which of the experiments here will the force acting on the rubber bung be greatest? And the answer is A, because it has the greatest mass hanging from it.

Well done if you picked that.

So now you're going to do the investigation.

You'll attach a rubber bung to a strong thread, thread that through a thin plastic tube, and attach a hanging mass from the other end.

Then you hold the plastic tube firmly in one hand and you spin the bung around until it follows a circular, horizontal path or very nearly horizontal path.

Rotate the bung in larger and smaller circles and observe any changes in how long it takes to make one complete orbit.

And then if you can, describe how the size of the orbit affects how long it takes to make one complete orbit.

Press pause when you do this activity and press play when you finish.

I hope you were able to see the effects of changing the radius of the orbit.

Here's what you would expect to see.

The bung takes longer to complete an orbit when the radius of the orbit is larger.

And the smaller the orbital radius, the shorter the time for each orbit.

And this is also true for satellites as they orbit the Earth.

When the radius of orbit is larger, the satellite takes longer to complete one orbit.

The International Space Station, which is in a relatively low orbit, about 400 kilometres above the Earth's surface, takes only about 90 minutes to travel around the Earth once.

For a satellite to be geostationary, taking 24 hours to circle the Earth, it needs to be much further away, about 36,000 kilometres above the Earth's surface.

And that's the end of the lesson.

So I'll finish with a summary.

Satellites are objects in orbit around a planet.

They can be natural or artificial.

Some artificial satellites are in low Earth orbits and can take detailed images of the Earth.

Those in polar orbits can travel over all of Earth's surface each day as Earth rotates beneath them.

Communication satellites are in geostationary orbits above the equator.

They orbit once each day so that they remain above the same point on the ground as Earth rotates.

Gravitational forces cause a satellite to follow a circular orbital path around the planet.

So well done for working through this lesson.

I hope you found it interesting, and I hope to see you again in a future lesson.

Bye for now!.