Lesson video

Hello and welcome to this lesson on gravity.

This is from the unit called Our Solar System and Beyond.

My name's Mr. Norris.

Now gravity is perhaps the most famous force that there is, and it's actually, it's the only force that really has an effect in space.

So within the solar system or between the stars or between different galaxies.

So if you want to understand space, you really have to have to understand gravity.

So that's what this lesson is aiming to do.

Let's start.

The outcome of this lesson is hopefully by the end of the lesson you'll be able to describe the effects of gravity, which acts towards the centre of a nearby planet, moon or star.

Here are key words that will come up this lesson, gravitational force, attraction and orbit.

Each word will be explained as it comes up in the lesson.

The lesson is divided into three parts.

In the first section we'll introduce the idea of gravitational attraction.

In the second part of the lesson, we'll look at the force of gravity on the moon and on other planets.

And in the third part of the lesson, we'll look at the effects of gravity within the solar system.

Let's begin by looking at the idea of gravitational attraction.

So gravitational forces pull objects towards large objects like planets, moons, and stars.

So you can see the purple arrow there representing a gravitational force acting on the skydiver because of Earth.

So we can call that the gravitational force of Earth on the parachutist.

Now, quite often we'll draw gravitational forces pointing directly down towards the Earth's surface, like in that diagram.

But that's actually just a different view from a different perspective of the same force.

And from this perspective, we can call that force the gravitational force.

We could call it a weight force or just weight or the weight of an object or weight force acting on an object.

So gravitational forces should never be labelled just as gravity.

They should be called gravitational forces or weight forces, or an object's weight.

Next thing to say about gravitational forces is that they're non-contact forces, and that's because the objects don't have to be touching for the force to act.

And that's like magnetic forces.

Magnetic forces are also non-contact forces.

You could see that those two magnets would attract even though they're not touching.

Well, that's exactly the same as the skydiver or parachutist and Earth.

The skydiver is attracted or pulled down towards Earth even though they're not touching Earth.

However, magnetic forces can cause magnets to attract or repel.

This picture shows two magnets attracting with two attractive forces.

And that second picture, which has just appeared below the line, shows two magnets, repelling.

You can see you've got a south pole and a south pole facing each other.

So those magnets repel.

There's two repulsive forces acting.

So that's different to gravity.

Gravitational forces only ever cause attraction towards the centre of the nearest planet, moon or star.

So gravity is only ever attractive.

Gravity only ever pulls objects closer together.

Okay, time for a check on what we've done so far.

This picture shows an apple falling from a tree in three stages.

Stage A, when the apple is still on the tree.

Stage B, when the apple is falling.

And stage C, when the apple is hitting the ground is no longer moving.

So at which points is a gravitational force acting on the apple.

So choose as many as you think from A, B, and C, at which points is a gravitational force acting.

Pause the video now and choose A, B, or C as many as you think.

Right, I'll give you some feedback.

So a gravitational force acts at point A when the apple is still on the tree because when the apple is on the tree, the force of gravity from the Earth is still pulling down on the apple.

It only doesn't fall because there must be a force from the branch which cancels out the downs force of gravity from the Earth.

But that down force of gravity from the Earth does still act when the apple is still on the tree.

When the apple is falling, gravity is acting there.

That's what makes the apple fall and get faster as it falls.

And at point C, when the apple is on the ground, well actually gravity is still acting at that point as well.

It's only not falling now because it's in contact with the ground.

So there's a normal contact force from the ground which pushes against the force of gravity so that apple can't go any further.

But the force of gravity is still acting on the apple, kind of keeping it in contact with the surface of the Earth.

So well done if you said, actually, yes, gravity, a gravitational force was acting on the apple at all three points during its fall.

Remember that gravity is a non-contact force.

So it acts whether objects are touching the Earth or not.

So Earth's gravity is what causes the attraction that pulls things towards Earth.

We could say Earth's gravity is what causes gravitational forces.

So here's a picture of Earth's gravity surrounding the Earth.

Earth's gravity extends far into space.

It doesn't stop at the end of these grey lines.

Imagine those grey lines continuing on outwards from the Earth.

So Earth's gravity extends far into space but gets weaker the further you get from Earth.

And a gravitational force then acts on any object that's within Earth's gravity.

So this object here, the skydiver is within Earth's gravity.

So a gravitational force acts on it, and you have it to be extremely far away from Earth to not really be classed as being within Earths gravity.

The next thing to say about Earths gravity is it always attracts or pulls objects towards the centre of Earth.

So on all of these objects, we've got the over the apple, the the bag being held by the person, the gravitational force that acts on all of them is pulling each object towards the centre of the Earth.

So this relates to like what the direction of down means.

The direction of down really means the direction that things fall.

So the direction of gravitational attraction, the direction things fall through air.

And of course that's a different direction on different parts of Earth's surface.

Okay, 'cause look at those arrows for the three different objects of different parts of the Earth in different countries, those arrows are all pointing in different directions in one sense, some of them are pointing down on the screen, some of them are pointing up on the screen.

However, in another sense that those arrows are all also pointing in the same direction, which is towards the centre of Earth, and that's the direction that gravity always attracts or pulls objects.

Let's do a check now about what we've just been talking about, about the direction gravitational forces pull the direction gravity pulls.

So which way does gravity pull on this climber? Is it direction A, direction B, direction C, or direction D? Take a moment to choose which option you think is correct, now.

Okay, now before I tell you which answer was correct, I now want you to justify your answer by choosing which one of these options is the best reason for which direction you chose for the direction gravity pulls on the climber.

So is it, did you choose your direction? Because gravity pulls towards the ground because gravity pulls towards Earth surface because gravity pulls towards Earth centre or 'cause gravity pulls objects upright.

So you need to finalise which direction you chose.

So that's the letters in the arrows, A, B, C, or D.

And you also need to finalise, which is the reason why you chose that direction from the four written A, B, C, or D.

Make sure you've made your two choices now.

Okay, I'm gonna go through the correct answers now.

So the correct direction the gravity pulls on the climber is direction C, but the correct reason for choosing direction C was reason C, which is a coincidence that they were both C.

So well done if you've got both of those.

The best explanation for why C is the correct direction that gravity pulls is because gravity pools object towards Earth centre.

That's the rule that you should try and remember about the direction gravity pulls.

Time now to do a task about gravitational attraction.

So the situation in this task is that a person is exploring a cave.

They're 100 metres under Earth surface, and they're surrounded by rock in in all directions.

Now look at the three pupils comments.

Asia says Gravity pulls towards Earth's surface, which is upwards here.

Alex says, gravity will pull them towards every surface.

So they'll float in the cave.

And Sam says, gravity pulls them downwards in the picture.

So two questions to write a response to, firstly, which pupil is correct and explain why they're correct.

And secondly, explain why the other two pupils are incorrect.

So have a go at doing that task now.

You actually need to write an answer to put your thoughts down into words on paper.

So have a go at doing that now with your best effort.

Off you go.

Well done for your effort with that task.

I'm gonna give you some feedback now, and I'm gonna give you some example answers.

So don't worry if your answers aren't identical to mine, if they're along the same lines, then that's great, but you could add anything to your answers to improve your work.

So question one, which pupil is correct? Explain why they are correct.

Sam was correct.

Gravity pulls the Caver downwards in the picture, and Sam is correct, 'cause gravity pulls objects towards the centre of the Earth and that would be downwards in the picture.

And then explain why the other two pupils are incorrect.

Well, the other pupils are incorrect 'cause people are not pulled upwards and they don't float around when they go into ground.

For example, if they go into a basement, a subway, or a railway tunnel, both of the other two pupils made the mistake of thinking gravity attracts objects to surfaces and that's not right.

That's a mistake students often make.

Gravity attracts objects towards the centre of Earth or towards the centre of the nearest planet, moon or star.

So well done if you got answers along those lines.

So that's the end of the first section of the lesson about gravitational attraction, attracting objects towards the centre of the nearest planet, moon or star.

Let's talk now about gravity on the moon and other planets.

So there are many differences between Earth and the moon.

Earth is a planet that orbits the sun, whereas the moon orbits around Earth at the same time.

It's the Earth orbits around the sun.

That's shown in the diagram.

Another difference is that Earth is much larger than the moon.

Those two drawings, there are about the right scale for the sizes of Earth and moon.

And another difference between Earth and moon is Earth has an atmosphere, air, but the moon does not.

There's no air around the surface of the moon, but there is air around the surface of Earth we're breathing it now.

However, Earth and the moon both have gravity, so that's not a difference.

There is gravity on the moon, just like there is gravity on Earth.

This represents the moon's gravity.

And that would represent the gravitational force of a person holding a bag on the moon or an astronaut holding a bag of tools, perhaps on the moon's surface.

The gravitational force points towards the centre of the moon.

Just like if they were on Earth, the gravitational force would point towards the centre of Earth.

However, the strength of gravity on the moon's surface is smaller than on the Earth's surface because the moon is much smaller and has far less mass than the Earth.

It's important to say photographs taken on the moon, shown that there's gravity there.

Have a look at this photograph that was taken on one of the Apollo missions to the moon in the '60s and '70s.

Gravity is clearly holding everything down in the picture.

Let's do a check now about what we've been talking about.

So an astronaut is walking on the moon carrying a hammer, which is true? So look at A, B, C, D, and E, and which is true? A, there is no gravity, only Earth has gravity.

B, there's no gravity because there's no air.

C gravity is stronger than on Earth.

D gravity is weaker than on Earth.

She can jump higher and falls back down more slowly.

E, if the astronaut, lets go of the hammer, it will just float next to her.

So choose which is true, five seconds to decide.

Off you go.

Well done if you said D.

All of the others are false.

There is gravity on the moon is just, its weaker than on Earth.

So astronauts can jump higher and would fall back down more slowly on the moon than on Earth because of the weaker gravity there.

So all very large objects have gravity, stars, planets, moons, dwarf planets, all very large objects.

So here's some representations of Mars' gravity and Neptune's gravity.

The larger the mass of an object, so basically the bigger the planet, the stronger its gravity will be at its surface.

And gravity is always strongest at the surface and slowly weakens with distance from the planet or from the moon.

So as an example of that, if you could take a bag to the surface of Mars, there would be a gravitational force acting on it because you'd be in Mars' gravity.

So there'd be a gravitational force pulling down on the bag.

That could be something like 75 newtons.

Whereas if you took the same bag to the surface of Neptune, now actually Neptune is a gas planet.

So whether it kind of has a surface or not is kind of up for debate.

But if you took it close to the surface of Neptune, that same bag would have a gravitational force of 220 Newtons pulling down on it.

So a far greater gravitational force acting on the same bag because it's somewhere where gravity is far stronger.

So a greater gravitational force would act on the same bag, on a greater mass planet due to the stronger gravity there.

Here is a check.

The sizes of the planets are shown to scale in this image.

Which planet do you think will have the strongest gravity close to its surface and why? Pause the video now, take a moment to decide and come up with a reason for your choice.

Off you go.

Now, based on what we just said about what makes a planet's gravity stronger, I hope you chose Jupiter.

Jupiter would have the strongest gravity close to its surface because it's the largest, it has the most mass.

Well done if you've got that.

Time to do a task now about gravity on the moon and on other planets.

So imagine that an object is taken to the surface of four planets, Venus, Earth, Mars, and Jupiter.

Look at the masses of each planet.

Venus is only 0.

8 of Earth's mass.

There's obviously Earth.

Mars is only 0.

1 of Earth's mass and Jupiter is 300 lots of Earth's mass.

So the first two things to do are to write prediction of how you think the weight of the object will compare on the different planets.

And then secondly, I want you to explain why you made that prediction.

Explain why you think the weight of the object would be different or similar or the same on the different planets that it's taken to.

Now, once you've done that, if you're in a classroom, your teacher might have bottles that are as heavy as the object feels on each planet.

So step three would be to pick up each bottle and compare the weight of the object on each planet.

So you'd be really getting as close as you can get to an experience of what the weight of the object would feel like, what the weight of the same object would feel like if it was taken to each planet.

And record what you found out.

Just put a few notes about how heavy each object felt at the surface of each different planet.

And then step four, identify if your prediction and explanation that you did in step one and two, were they correct? See if you can write down a better explanation.

Now you've had the experience of picking up each bottle and comparing what the weight of the object would feel like on each planet.

So have a go at all four stages of that task.

Pause the video now.

Go back to look at the slide with step one and step two if you need to before doing step three and step four.

Pause the video now.

Off you go.

Well done, for your effort on that task.

I'm gonna give some feedback on the written parts of that task.

So in part one, you were asked to make a prediction for the weight of the object on each planet.

And in part three, if you had the objects available to pick up, you were asked to compare how their weight felt different on different planets using the objects that your teacher might have had available.

And this is what you should have predicted for part one or found out in number three.

So you should have predicted or found out that the object felt lightest on Mars, then a bit heavier on Venus and Earth.

The object's weight would've been similar on Venus and Earth, just slightly heavier on Earth.

And the object should have felt much heavier on Jupiter.

And then for step two, you were asked to explain why you made that prediction or why you think that happened when you did the test.

And that's got quite a simple answer really, on planets with more mass, the gravity is stronger.

So the gravitational force pulling the object down would be greater, making the same object feel heavier on the planet with more mass, such as Jupiter, the object would've felt lightest on the planet with the smallest mass, which would have the weakest gravity that's mass.

The object felt heaviest on the planet with the greatest mass Jupiter, which would have the strongest gravity.

So well done if your answers, predictions and explanations were along those lines.

So we're now onto the final section of the lesson, which is on the effects of gravity in our solar system.

So we're gonna start this section of the lesson by talking about a slightly different situation of swinging an object on a string above your head in a circle, causing it to orbit.

A bit like what happens in the solar system actually, but we'll come to that in a minute.

So what causes something to move in a circle when it swung on a string above your head? Well, the answer is there's a tension force that comes from the string and that acts on the object.

What does that tension force do? Well, remember that forces can change the direction of objects and that's what the tension force from the string is doing.

In this case, the tension force is changing the direction of the object's movement.

Every moment in time the direction is changing and that's what pulls it round in a circle so it orbits and so it doesn't fly off unless of course the string breaks or unless you let go of the string and then the object will fly off.

But whilst that tension forces there, it keeps it moving in a circle by changing the direction of the object's movement.

And also notice how the tension force points in the direction towards the centre of the circular path that the object moves in.

So the tension force isn't pushing the object along the circular path.

The tension force is pulling the object in changing its direction.

That's what gives it the circular path.

That's an important difference.

So why have we been talking about swinging something around your head in a circle? Well, it's because the moon orbits Earth in an almost perfect circle as well.

But in this case it's Earth's gravity that provides the force that causes the moon to orbit.

So the curved arrow there shows the path of the moon around the Earth.

But this arrow that's just appeared shows the gravitational force of Earth on the moon.

And that's very similar to that tension force that was keeping the object on a string in orbit around your head.

The gravitational force is what keeps the moon in orbit around Earth.

So the gravitational force from Earth changes the direction of the moon's movement at every moment in time.

So at no point in time does the moon continue in a straight line in the same direction that it's already going.

At every point in time, the moon is changing direction and following a curve path in a circle around the Earth and that's because of that gravitational force of Earth on the moon.

So this is what makes the moon move around Earth in a circle.

So it orbits Earth and cannot escape.

Tick, the four statements that are true in this list, there's a lot to read through.

So you're looking for four statements that are true.

A, the moon's orbit of Earth is caused by gravity.

B, the moon is within Earth's gravity.

C, the moon has its own gravity.

D, the gravitational force from Earth pushes the moon along its orbit.

E, gravity pulls the moon in the direction towards Earth.

F, gravity pulls the moon closer to Earth.

Which do you think are correct? You need to choose four that you think are correct.

Have a go now.

Pulls the video and make your decision.

Right.

Let's see how you got on.

So the moon's orbit of Earth is caused by gravity and the moon is within Earth's gravity.

That's why gravitational force from the Earth can act on the moon.

The moon also has its own gravity, but we've not been talking about that right now.

That's just a separate fact.

So that leaves one more.

That is true and it is E.

Gravity pulls the moon in the direction towards Earth 'cause just like we said in the first part of the lesson, gravitational forces always point towards the centre of the nearest planet.

In this case, the nearest planet is Earth.

So the gravitational force from Earth on the moon is gonna pull the moon towards the centre of the Earth.

So D is incorrect.

The gravitational force from Earth does not push the moon along its orbit.

It pulls in a different direction towards the centre of Earth, changing the direction of the moon.

And F was also wrong.

Gravity actually doesn't pull the moon any closer to Earth.

Gravity, the force of gravity, the gravitational force from the Earth causes the moon to orbit Earth in a circle.

So it never actually gets closer to Earth.

It always stays the same distance away.

So well done if you've got all four of those.

Now, let's talk about the sun.

So this shows the sizes of the sun and Earth to scale.

So there's the sun, and that would be the size of Earth.

Now of course, the distance between the sun and the Earth is not to scale 'cause the distance between the sun and Earth's orbit should be 42 widths of the screen, however big the screen is that you are looking at, 42 of those screens would be needed to show the true distance between the sun at this size and the Earth of that size.

So that's a reminder about the scale of objects in the solar system.

And because of that huge difference in size between the sun and Earth, the sun's mass is around 300,000 times Earth's mass.

The sun is absolutely huge compared to Earth.

And what that means is the sun's gravity is far stronger than Earth's gravity or Jupiter's gravity or any planet's gravity, the gravity of anything else in the solar system.

So the sun has got incredibly strong gravitational pull 'cause of its huge mass.

And also the sun's gravity extends very, very far into space.

Everything in our solar system is within the sun's gravity.

So experiences a gravitational force towards the centre of the sun.

That's what gravitational forces do.

They pull towards the centre of the object that's providing that gravitational force.

So everything, the solar system experiences a gravitational force towards the sun.

However, things are not pulled closer to the sun because they've already got their own motion.

So the sun's gravitational force just pulls the objects round in a circle and ensures they orbit the sun.

So the gravitational force from the sun is why everything in our solar system orbits the sun for any object in our solar system, the gravitational force from the sun is changing the direction of its movement.

This is what we've said about everything that's been moving in a circle in this part of the lesson, the force that points towards the centre of the circle.

So in space that's a gravitational force changes the direction of objects movement, and that's what keeps them in orbit.

This makes each object move in a circle so it continuously orbits and cannot escape.

Fill the gaps to explain what causes Earth to orbit the sun.

Pause the video now and have a go at filling those gaps to give the key explanation from this part of the lesson.

Okay, let's see if you could remember it.

The key explanation from this part of the lesson is about why objects orbit in the solar system.

That's because of gravity or it's because of gravitational forces.

So it's a gravitational force from the sun changes the direction of Earth's movement, and that makes Earth move in a circle around the sun.

So it continuously orbits the sun and cannot escape.

That's how gravitational forces cause orbits, that explanation there.

So try and commit that explanation into your memory.

And a final part of this check for understanding which diagram correctly shows this force that causes Earth to orbit the sun.

Is it diagram A, diagram B or diagram C? Choose now, five seconds.

I imagine you will have all said diagram A that shows the direction of the gravitational force that causes Earth to orbit the sun.

Diagram B shows the path Earth takes not the direction of the force.

And diagram C shows the direction of Earth's movement at that point in time.

This is the direction that the gravitational force shown in a changes.

So time for the final task in this lesson.

You need to discuss with the people around you whether each statement is correct or incorrect.

And then tick the box that shows what you think.

So statement A, there's no gravity in space.

What do you think? Statement B.

The sun has no gravity because the sun has no air.

What do you think? Statement C, Saturn has stronger gravity than Earth does.

What do you think? And statement D, the sun's gravity stops the planets from escaping their orbits.

What do you think and why? So pause the video now and have a good think about each statement make sure you've got a reason for why you think each statement is correct, why you think it's incorrect.

And I'll see you in a few moments for some feedback.

Pause the video now.

Well done for your efforts on doing that task really well, here's some feedback.

There's no gravity in space.

That's incorrect.

Well done if you spotted that.

There's gravity around all objects in space, and the bigger the object, the more mass it has, then the stronger the gravity around it.

Statement B, the sun has no gravity because the sun has no air while the sun does high gravity.

So that statement was incorrect.

Well done if you got that.

Statement C, Saturn has stronger gravity than Earth does.

That's correct because Saturn is a larger mass planet, a bigger planet than Earth is.

So it's got stronger gravity.

Well done if you've got that.

And statement D, the sun's gravity stops the planets from escaping their orbits.

That's correct.

The sun's gravity and gravitational forces from the sun.

That's what keeps the planet in orbit around the sun.

So well done if you got that.

A final well done for your effort this lesson.

Here's a summary.

Gravitational forces attract or pull objects towards the centre of large objects like planet, moons and stars.

The greater the mass of an object, the stronger its gravity and the gravity of a planet moon or star extends far into space but gets weaker with increasing distance.

Earth's gravity pulls the moon towards the centre of Earth.

This changes the direction of the moon, making it orbit around Earth.

And the sun's gravity pulls planets towards the centre of the sun.

This changes the direction of the planet, making them orbit around the sun in a circle.