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Gravity in Space
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About the lecture
In this mini-lecture, we discuss gravity in space, focusing in particular on: (i) a demonstration that involves wringing out a wet washcloth in the International Space Station that is orbiting around Earth (done by astronaut Chris Hadfield); (ii) Newton’s Cannon, a thought experiment that helps us understand orbits and the concept of freefall; (iii) a video of an astronaut bunny hopping on the Moon; (iv) gravity on Earth (10 N/kg) compared to gravity on the Moon (1.6 N/kg); (v) the relationship between mass and gravitational field strength; (vi) orbits, which involve an acceleration due to the change in direction of the orbiting object; and (vii) a demonstration of the relationship between radius and velocity of an object in orbit.
About the lecturer
Dr Pete Edwards is the Director of Science Outreach and the Science and Society Officer at Durham University. He is also a member of Durham University’s Astronomy and Astrophysics group. As the Director of Science Outreach at Durham University, Dr Edwards has developed a range of teaching resources aimed at primary and secondary students and teachers. He has visited one in three secondary schools in the UK, where he has provided a programme of physics demonstrations and talks. As an elected fellow of the Institute of Physics (IOP), he has contributed to the Teaching Astronomy and Space DVD (2010-2019) produced by the IOP, as well as the IOP videos How Big is the Universe? (2012) and The Expanding Universe and the Big Bang (2012). In 2005, he was chosen to deliver the 2006 IoP Schools’ and Colleges’ Lecture. This talk, Gravity, Gas and Stardust, was broadcast across the UK to over 12,000 14-16 year old students. He has also worked with TED to create the TED-Ed video, What light can teach us about the universe (2014). Dr Edwards is also a qualified secondary school teacher, who taught science and mathematics in various schools and colleges during the 1980’s before obtaining his Ph.D. and joining Durham University in 1990.
Hi. So hello. So my name is Dr Pete Edwards. I work at the University of Durham
00:00:06and I'm an astronomer here. And today we're going to talk about astronomy.
00:00:11So let's start.
00:00:16We know
00:00:19there's gravity on the Earth,
00:00:21but is there gravity in space?
00:00:23Okay, so here's my washcloth.
00:00:26Like a magic trick.
00:00:28And now I'm gonna get this soaking wet,
00:00:30and then we're going to see what will happen when we wring it out.
00:00:32Mary, Kate and Kendra suggested that I dip this in a bag,
00:00:37but bags don't hold water in space.
00:00:41So instead I filled the water bag. This has drinking water in it,
00:00:43and I'm going to squirt a bunch of water into this washcloth.
00:00:49Okay,
00:01:12so here's a soaking wet washcloth.
00:01:12Get the microphones. You can hear me while I'm talking,
00:01:16and now let's let's start wringing it out.
00:01:18It's really wet.
00:01:23It's becoming a tube of water.
00:01:40The water's all over my hands. In fact, it rings out of the cloth into my hands.
00:01:46And if I let go of the cloth carefully,
00:01:49the water sort of has it sticks to my hand.
00:01:54Okay, so the experiment worked beautifully, and the answer to. The question is,
00:01:58the water squeezes out of the cloth
00:02:03and then, because of the surface tension of the water,
00:02:06it
00:02:09it actually runs along the surface of the cloth and then up into my hand,
00:02:10almost like you had
00:02:15Jell O on your hands or gel in your hand.
00:02:16And it'll just stay there.
00:02:19Wonderful
00:02:20moisturiser on my hands
00:02:21and the cloth doesn't really unravel itself.
00:02:23It just stays there, floating like a uh like a dog's chew toy soaking wet.
00:02:26Great experiment
00:02:32worked perfectly.
00:02:33MEREDITH and Kendra. Congratulations. Great idea.
00:02:35So from looking at the video of Chris, it's obvious that there is no gravity in space.
00:02:38But that's just not correct.
00:02:43When you see the videos of guys
00:02:46floating around in the International Space Station, it's quite obvious that,
00:02:49you know, it's an obvious conclusion to draw that there is no gravity up there.
00:02:53But that's not, in fact, the case.
00:02:57And there's a thought experiment that Isaac Newton first came up with
00:02:59to sort of explain what's going on when something orbits the earth.
00:03:02So in his experiment, Newton visualised a cannon on top of a very high mountain,
00:03:05the cannons firing a cannonball and If there
00:03:11are no forces of gravity or air resistance,
00:03:13the cannonball should just follow a straight line and fly away from the Earth.
00:03:16If gravitational forces act on the cannonball,
00:03:20it will follow a different path depending upon the speed of the cannonball.
00:03:23If the speed is low,
00:03:28it will simply fall back to Earth shown as a and B and this diagram
00:03:29and that actually for the earth any velocity between between zero
00:03:34and 7000 metres per second,
00:03:39the cannonball would eventually fall to the Earth.
00:03:41If the speed is just right, though,
00:03:44the cannonball is falling towards the earth. But it never actually gets there.
00:03:47You can think of the cannonball is falling towards the earth,
00:03:52but the surface of the earth moving away from the cannonball at the same rate.
00:03:55And that gives us what's called an orbit. And you can see that as see in the diagram
00:03:59and for the earth that if you fired a cannonball
00:04:04that approximately a speed of about 7300 metres per second,
00:04:07it would all bit the earth
00:04:11if the cannibals speed is higher of course
00:04:14than this orbital velocity,
00:04:16but not high enough to leave the earth all together
00:04:19what we call the escape velocity.
00:04:22Uh then the ball will continue revolving
00:04:24around the earth along an elliptical orbit.
00:04:27The orbit wouldn't be circular. It would be an elliptical shaped like an egg shape.
00:04:29But the ball, the cannibal would still orbit around the earth
00:04:34and that those speeds are roughly, what,
00:04:387300 metres per second to about 10,000 metres per second.
00:04:40And of course, if the speed is very high,
00:04:46the cannonball would leave the earth in a parabola
00:04:48and you'd never get it back.
00:04:52So
00:04:55there is gravity acting in space.
00:04:56When you're in the international space station,
00:04:58you're actually falling towards the earth.
00:05:00Gravity is pulling
00:05:03everything down in the international space station,
00:05:04but they're all falling at the same rate.
00:05:06It's what we call free fall.
00:05:09So if you're inside the space station,
00:05:11everything around you is falling with you at the same speed towards the earth.
00:05:13And so it would appear that there's no gravity acting.
00:05:17But we now know, of course, there is gravity acting,
00:05:20keeping the space station and its contents
00:05:23in orbit around the earth.
00:05:25What about other planets? Is there gravity on the moon, for example.
00:05:29Be warned. When you do this, then you go down slope.
00:05:38That first step is a long one
00:05:41I'm having.
00:05:47This is the best way for me to travel uphill or downhill
00:05:53a track like this to like it hot
00:05:58down on level ground I can skip.
00:06:02I don't like that Lupin thing.
00:06:05It'll open is the only way to go.
00:06:08Check point
00:06:10here on my level ground I can.
00:06:11But this two legged thing is great
00:06:14and I've covered ground like a kangaroo.
00:06:17So what's going on on the moon?
00:06:22You can see that there is gravity on the moon,
00:06:23the astronauts bunny hopping around as they called it.
00:06:26But obviously the strength of gravity on the moon is less.
00:06:29And we can explain that
00:06:32because the gravity of any object the amount of gravity any object produces
00:06:35depends upon the mass of that object.
00:06:40So on the earth,
00:06:42the gravitational field strength the downward force on a one kilogramme masses.
00:06:4410 Newtons.
00:06:50And that's called the gravitational field strength. What we call little G actually
00:06:5210 Newtons per kilogramme.
00:06:56So the moon has a smaller mass than the earth.
00:07:00And so the pull of gravity by the moon on any object is smaller
00:07:04on the moon, the gravitational field strength is only 1.6 Newtons per kilogramme.
00:07:08In other words, one kilogramme mass has a weight on the moon of 1.6 Newtons,
00:07:14whereas on the earth it's 10 Nunes.
00:07:18So you can see there. The astronauts are hopping around
00:07:20in a lower strength field.
00:07:24The gravitational field strength of any object depends upon its mass,
00:07:26so more massive object has a stronger gravitational field strength.
00:07:30Less massive object has a smaller one,
00:07:34so we talked about orbits in space.
00:07:39There are many things orbiting many other things.
00:07:43Planets like the Earth orbit the sun. The moon orbits the Earth.
00:07:46Comets orbit the sun to and move through the solar system.
00:07:50So the idea of Orbitz is an interesting one.
00:07:55Um, we saw from the Newton's cannonball experiment that
00:07:58the force of gravity is actually the force that keeps
00:08:03the object moving in an orbit.
00:08:07If something has a force acting on it.
00:08:10Newton's second law tells us that it must be accelerating.
00:08:13But when you think about the orbit of a body,
00:08:17its speed isn't changing. It's still moving around the sun.
00:08:20The earth still moves around the sun
00:08:23at the same speed.
00:08:25It doesn't change, So how can it be accelerating?
00:08:26The answer lies in the fact that acceleration is
00:08:30actually a change in the velocity of an object.
00:08:32Velocity is a vector quantity,
00:08:35so velocity is defined by the speed and the direction in which an object is moving.
00:08:37So even though when you're in orbit, your speed isn't changing,
00:08:43your velocity is changing because the direction
00:08:46in which your body is moving is changing.
00:08:49When you're in orbit, your speed doesn't change, but your velocity is changing.
00:08:52In other words, you are accelerating.
00:08:57If you change the radius of the orbit, the velocity of the body speeds up.
00:09:02And this is something that Kepler first thought about when he was looking at
00:09:08the motions of planets in our solar system a few 100 years ago.
00:09:12So here we have the ball in a stable orbit,
00:09:16and hopefully you'll see
00:09:21as I
00:09:23make the orbit radius smaller,
00:09:24the ball
00:09:27speeds up.
00:09:28Is that right?
00:09:32So we've just talked about gravity and space
00:09:33and how things orbit other things.
00:09:36But now let's think about what's out there in space. What's doing the orbiting
00:09:38
Cite this Lecture
APA style
Edwards, P. (2022, January 14). Module 8: From the Universe to the Atom - Gravity in Space [Video]. MASSOLIT. https://massolit.io/options/module-8-from-the-universe-to-the-atom?auth=0&lesson=4523&option=16829&type=lesson
MLA style
Edwards, P. "Module 8: From the Universe to the Atom – Gravity in Space." MASSOLIT, uploaded by MASSOLIT, 14 Jan 2022, https://massolit.io/options/module-8-from-the-universe-to-the-atom?auth=0&lesson=4523&option=16829&type=lesson