Date: October 1st, 2012
Title: Gravity by Astronomy For Kids
Podcaster: Jeffrey Tang
Organization: Astronomy For Kids
Links: www.astronomyforkids.com.au
Description: In this podcast created especially for kids we talk about gravity. We have a look at what we know about gravity and discuss the effects of gravity on us and on the universe.
Bio: Astronomy For Kids is an astronomy podcast designed especially for kids. It is presented by Jeffrey Tang who is a kid himself. Our aim is to encourage kids to be interested in science. We created the podcast because Jeffrey was interested in astronomy but we could not find a podcast that was at a suitable level for him.
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Transcript
Hi and welcome to the 365 Days of Astronomy Podcast for the 1st of October, 2012. Today we will be presenting an episode of Astronomy For Kids, a podcast created especially for kids interested in astronomy. We hope you enjoy this episode.
Astronomy For Kids Podcast Episode 5: Gravity
Welcome to the Astronomy For Kids podcast. I’m Jeffrey Tang. In this podcast we are going to talk about something very important in our everyday lives, gravity.
But before we talk about gravity, we should really talk about mass, as mass is very important to gravity. The mass of something is a measure of how much stuff it is made of. In everyday life, we usually think of mass as meaning weight. When we weigh ourselves, that gives us our mass or weight.
But our weight actually measures how much our bodies are pushing down on the earth because of the effect of gravity. If we were to go to the moon, our mass would not change because we are still made of the same amount of stuff. But our weight would change because there is less gravity on the moon. Because the gravity on the moon is less, it does not pull us towards the moon with same amount of force, so on the moon we would weigh about one sixth as much as we do on earth. This is the same for other objects. If we weigh an object on the moon, it will weigh much less than on Earth, but it will still have the same mass because it is still made of the same amount of stuff.
The reason that mass is so important to understanding gravity is that the amount of gravity that an object produces depends on its mass. Gravity causes objects to want to move towards one another. The more mass an object has, the bigger the effect of its gravity.
The reason that all of us here on earth don’t go floating off into space is that the Earth’s gravity pulls us towards it. At the same time, the gravity of our bodies also pulls the Earth towards us, but only a tiny, tiny bit. Gravity holds our atmosphere in place around the earth and gravity causes things fall to the ground when we drop them.
Because different planets and moons have different masses, their gravity is different. Some do not have enough gravity to hold onto an atmosphere. Other planets with more mass have much more gravity. If we could land on a planet with a lot more gravity, we would weigh a lot more and we would find it very difficult to move around. It would take much more effort to lift our feet off the ground.
Gravity has such a big effect throughout the whole universe, not just here on Earth. The reason that all of the planets in our solar system orbit the sun is because of gravity. If the planets weren’t being pulled towards the sun by gravity, they would just travel in a straight line and go wandering off into the galaxy. And guess what? The galaxy is held together by gravity too.
All of the stars and planets in the universe formed because of gravity. All of the particles of stuff that make up the stars and planets came together because gravity made them move towards one another and clump together.
Scientists have spent a lot of time trying to understand gravity. The first person to come up with a well known theory of how gravity works was Sir Isaac Newton, in 1687. Newton thought of gravity as a force that makes all particles of stuff attract all other particles of stuff. This attraction means that the particles want to move towards one another. Newton worked out that the strength of gravity created by an object depends on its mass and also, on how far away it is. Newton’s theory of gravity was used for a long time and it is still good enough to be used in many situations, like rocket launches.
In 1916, Albert Einstein came up with a new way of thinking about gravity. Einstein thought that objects with mass actually push on space causing a dip. Other objects then fall into this dip and that’s what makes things move towards one another.
So far, Einstein’s theory of gravity is the best one that we have. From all of the tests that scientists have done and all the things that have been seen in the universe, Einstein’s theory of gravity fits most situations. The problem is that when we get down to the tiny particle level, Einstein’s theory does not explain some of the things that scientists see. So we are still waiting to find a complete theory of gravity that works for all situations.
Let’s take a look at some examples of things in the universe that have very cool effects of gravity. One type of object that we have found that create a huge amount of gravity are black holes. There are two main types of black holes, stellar mass black holes and supermassive black holes.
Supermassive black holes are enormous black holes in the centres of galaxies. It seems that just about all galaxies have supermassive black holes at the centre. Some of these can have the same mass as billions of suns. Stellar mass black holes are much smaller than supermassive black holes. They have a mass between about three times and twenty times the mass of our sun.
Anything that gets too close to a black hole gets pulled in and squashed up. The gravity of black holes is so strong that even light cannot get away from it. The light that we see coming from the area of a black hole is actually from all the stuff around the black hole releasing energy as it is being pulled in by the gravity and smashed together. We see stellar mass black holes when they slowly eat up another star. We see supermassive black holes when they gobble up the gas, stars and planets around them. We see, this energy with our different types of telescopes and this tells us that there is a black hole there eating stuff up. No one knows what happens to the stuff once it gets inside a black hole, but it must stay inside because the mass of a black hole increases when stuff goes ino it.
Although these black holes have very strong gravity, they do not suck everything into them. As long as objects are far enough away from a black hole, they will just orbit around it as though it was a star. Black holes only suck in objects that get close so we don’t have to worry about being sucked into the black hole at the centre of our Milky Way Galaxy.
Gravity acts over very long distances, so sometimes two whole galaxies can be pulled together because they have a huge mass.
Our Milky Way Galaxy is part of what we call the local group. The galaxies in this group are held together in a bunch by gravity. In fact, gravity is causing the galaxies to move closer together. The Andromeda Galaxy is the biggest galaxy in our local group and is a spiral galaxy like our Milky Way. Even though the Andromeda Galaxy is so far away from us that I can’t even say the number of kilometres, gravity is still causing our galaxy and the Andromeda Galaxy to move towards one another. In about 4 billion years they are going to come together and our Solar System will become part of a giant elliptical galaxy. I think it’s pretty amazing that gravity can still have an effect over that enormous distance.
I hope this episode has given you something to think about.
If you would like to contact us, you can go to our website at www.astronomyforkids.com.au or send an email to astronomyforkids@hotmail.com.au. This podcast was presented by Jeffrey Tang, written by Susan Tang. Thanks to our Science Advisor, Mark Purver. Post production by David Kelly and Susan Tang. Until next time, bye.
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