Brian Cox: Gravity is the force that keeps our feet firmly rooted to our planet.
Yet although it may appear constant and unchanging, this force varies on all the planets in the solar system and on the exoplanets we've discovered orbiting other suns.
To experience the gravity on these worlds I need to go for a spin.
ELECTRONIC PULSING MUSIC
This is a centrifuge. It was built in the 1950s to test whether fighter pilots had the right stuff.
But it's gonna rely on me to feel what it'd be like to stand on the surface of any of the planets in the solar system that are more massive than the Earth, and in fact, also, what it would be like to stand on some of the planets that we've found around distant stars.
Man: Three. Two. One.
Brian Cox: As the centrifuge rotates, it feels exactly as if gravity has increased.
The faster it spins the greater the effect.
And we measure this in multiples of the strength of Earth's gravity, known as 1G.
The first planet I'm travelling to is Neptune.
Its gravity is just fractionally stronger than here on Earth.
So this is the gravitational field on Neptune. And you feel, you know what, I could probably get used to this. I could probably live on the surface of Neptune.
Man: Can- can you lift your hands a little?
Brian Cox: There we go.
Man: Yeah. And down?
Brian Cox: And it is actually quite an effort, it is noticeably heavier. It's like having a reasonably heavy weight in your hand.
Man: To go to two-and-a-half G?
Brian Cox: Yes. And I will move, move from Neptune to Jupiter.
Man: Let's go there.
Brian Cox: Jupiter is over 1300 times more massive than the Earth.
But because it's mostly gas it's not very dense, so its gravity is just over twice as strong at its surface. Well now, actually, it is quite difficult to lift my hand.
And that's 2.5G. I wouldn't wanna sit here for half an hour.
Man: Can you lift both of your hands above your head? See what happens there.
Brian Cox: Let's see. So, actually…just about. But actually it's, er, an immense amount of hard work.
So it would be hard work living on Jupiter!
Man: Let's go to 4G.
Brian Cox: Actually, this is heading to a- a planet around a planet called OGLE2-TR-L9b, which is around a star in the constellation of Carina.
It's one of the exoplanets we've discovered.
Oh, and there we go.
Now, that is actually beginning to feel quite unpleasant.
Man: Can you describe what you're feeling?
Brian Cox: A very heavy face.
Man: All right.
Brian Cox: My head is extremely heavy.
Man: How about your lungs? Inhaling, exhaling? Breathing?
Brian Cox: It's much harder work. I can't lift my hand off my leg.
Man: Okay.
Brian Cox: And that's at 4G?
Man: Yeah.
Brian Cox: Well, my head and my face feel very, very heavy.
Man: Yeah.
Brian Cox: It's quite an unpleasant feeling.
Man: We'll go to 5, and let me know if you have any visual disturbances.
Brian Cox: I'm now en route to a newly discovered exoplanet, WASP-8b. 4.4.
This world sits in the small and feint constellation of Sculptor.
Quite hard to speak.
It has a gravitational force nearly five times that of the Earth.
Man: All right, we'll go to 5G.
Brian Cox: Oh, very foggy.
Man: Okay.
Brian Cox: Very foggy.
Man: Very foggy?
Still foggy?
Brian Cox: Yeah.
Man: All right.
Brian Cox: Take it down.
Man: Okay, we'll take you down.
Brian Cox: Very interesting.
Man: It was, wasn't it?
Brian Cox: My face felt a bit saggy there.
MAN CHUCKLES
Man: Yeah. Well, you looked a little different.
Brian Cox: That was, er, quite unpleasant that time actually.
So, you realise that we're, obviously, very finely tuned to live on a planet that has a gravitational- an acceleration due to gravity of 1G. When you go to 2G it's difficult. When you go to 3G and 4G it becomes unpleasant.
And 5G, anyway for- for me, was, er, on the border of being, er… so unpleasant that you pass out.
So, although gravity feels weak here on Earth… it certainly isn't weak everywhere across the universe.
And that's because gravity is an additive force. It scales with mass.
So, the more massive the planet or star, the stronger its gravity.
Professor Brian Cox simulates the strength of gravity on other planets using a centrifuge in Holland.
Gravity is the force that keeps our feet on the ground.
Although it may appear constant and unchanging, this force varies on all the planets in the solar system and on the exoplanets orbiting other stars.
The greater the planet's mass, the greater the force of gravity.
Brian gets inside the centrifuge to experience what it would be like to stand on the surface of Neptune, Jupiter, OGLE2-TR-L9b and WASP-8b, which has a gravitation force 5 times that of Earth.
This clip is from the series Wonders of the Universe.
Teacher Notes
Introduce the relationship between gravity and weight with this clip.
This could be modeled by having sealed boxes containing masses which give the feel of what 1kg would feel like on different planets.
This could then lead on to worked examples of the formula used for the calculations before students can carry out their own.
Curriculum Notes
This clip will be relevant for teaching Physics.
This topic appears in OCR, Edexcel, AQA, WJEC KS4/GCSE in England and Wales, CCEA GCSE in Northern Ireland and SQA National 4/5 in Scotland, and Cambridge IGCSE Physics.
