What is gravity and why is it so important? 9J Gravity and space What is gravity and why is it so important? Pearson Education Limited 2004

9J You should know… 9Ja What gravity and weight are and how they are measured. 9Jb How gravity changes depending on where you are. 9Jc How our ideas about the Solar System have changed. 9Jd The importance of gravity for planets, satellites and spacecraft.    Pearson Education Limited 2004

You should know… What causes weight. Which direction ‘down’ is. 9Ja You should know… What causes weight. Which direction ‘down’ is. How gravity affects objects. How to work out the weight of an object. Pearson Education Limited 2004

9Ja Gravity All objects exert a small force on other objects which tries to pull them together. The force is called The bigger the mass of an object, the greater its gravitational pull. gravity. The Earth has an enormous mass. Its gravity is very and acts downwards, towards the centre of the Earth. Pearson Education Limited 2004

9Ja Mass and weight Weight and mass are connected, but they are not the same thing. Mass is the amount of matter in an object. It is measured in kilograms (kg). The force of gravity pulling on an object’s mass is its weight. Weight is measured in newtons (N). Pearson Education Limited 2004

Mass and weight The force of gravity on Earth is 10 N per kg. 9Ja Mass and weight The force of gravity on Earth is 10 N per kg. This man’s mass is 90 kg, so his weight is 90 x 10 = 900 N. This boy’s mass is 45 kg, so his weight is 45 x 10 = 450 N. Pearson Education Limited 2004

9Ja Sir Isaac Newton The unit of weight is named after Sir Isaac Newton, who lived from 1642 to 1727, and carried out important work on forces. Newton worked out how to calculate the force of gravity on objects. A falling apple is said to have inspired Newton to create his laws of gravity when it fell on his head. It has a weight of about one newton. Pearson Education Limited 2004

9Ja Try these… Unscramble these letters, and then explain what the word means. t i v y a r g _ _ a v _ _ _ gravity An attractive force possessed by all objects that have mass. Pearson Education Limited 2004

Unscramble these letters, and then explain what the word means. 9Ja Unscramble these letters, and then explain what the word means. g e w h i t _ _ i g _ _ weight The force of gravity acting on the mass of an object. Pearson Education Limited 2004

Unscramble these letters, and then explain what the word means. 9Ja Unscramble these letters, and then explain what the word means. w e n n o t _ _ w t _ _ newton The unit of weight, named after Sir Isaac Newton. Pearson Education Limited 2004

You should know… Why gravity is different on other planets. 9Jb You should know… Why gravity is different on other planets. How to calculate weight on other planets. How gravity changes with distance. How changing gravity affects spacecraft. Pearson Education Limited 2004

Gravity on the Moon The Moon is about one sixth the mass of the Earth. 9Jb Gravity on the Moon The Moon is about one sixth the mass of the Earth. If you went to the Moon, your mass would stay the same but your weight would decrease. Your mass does not change because you are still made up of the same amount of matter. Your weight changes because the Moon’s gravitational pull on your body is less than that of Earth. Pearson Education Limited 2004

9Jb Gravity on the Moon A B Twin A on Earth has a mass of 60 kg and a weight of 600 N. Twin B on the Moon has a mass of 60 kg and a weight of 100 N. Pearson Education Limited 2004

9Jb Gravity in space The force of gravity gets less as objects move further apart. As a spacecraft gets further away from the Earth, the pull of the Earth’s gravity on it gets less. At a certain point between the Earth and the Moon, the pull of the Earth is cancelled out by the pull of the Moon. Pearson Education Limited 2004

Try these… True or false? 9Jb Try these… True or false? 1. You weigh more on the Moon than on the Earth. False You weigh less on the Moon than on the Earth, because the Moon’s gravity is about one sixth that of the Earth’s. Pearson Education Limited 2004

9Jb True or false? 2. The weight of an object depends on its mass and the force of gravity. True Pearson Education Limited 2004

3. You weigh less at the top of a mountain than you do at sea level. 9Jb True or false? 3. You weigh less at the top of a mountain than you do at sea level. True Pearson Education Limited 2004

9Jb True or false? 4. On the Moon, your weight is the same but your mass is less than on Earth. False Your mass is the same wherever you are, but your weight is less on the Moon than on Earth. Pearson Education Limited 2004

You should know… About some early ideas about the Solar System. 9Jc You should know… About some early ideas about the Solar System. How our modern ideas are different to the early ideas. Our current model of the Solar System. Some of the evidence for our current model of the Solar System. Pearson Education Limited 2004

9Jc The shape of the Earth The ancient Egyptians believed that the Earth was a flat plain with the River Nile running through the middle. The Babylonians thought the Earth was a large hollow mountain floating in water. Most people have thought of the Earth as a sphere for the last 2000 years. Pearson Education Limited 2004

Ideas about the Solar System 9Jc Ideas about the Solar System Aristotle and Ptolemy both had similar ideas about the Solar System. They thought that the Earth was at the centre and didn’t move, and the Sun and planets circled round it. Observations about the positions of the planets on different dates didn’t fit these ideas. Pearson Education Limited 2004

Ideas about the Solar System 9Jc Ideas about the Solar System Copernicus produced a better model, with the Earth going round the Sun, but it still wasn’t accurate and was very complicated. Earth The Earth orbits around this point. The centre of the Earth’s orbit is moving around this point, which is itself moving around the Sun. Pearson Education Limited 2004

Ideas about the Solar System 9Jc Ideas about the Solar System Our modern model of the Solar System was first suggested by Kepler in 1609. He discovered that the speed of Mars changed as it moved around its orbit. He suggested that the planets moved in ellipses, not circles. We still use this model today. It is used to calculate where the planets are and the journeys of spacecraft. Pearson Education Limited 2004

An elliptical orbit planet 9Jc An elliptical orbit planet The planet is moving fastest when it is at this part of its orbit. Sun The planet is moving slowest when it is at this part of its orbit. Pearson Education Limited 2004

Try these… They believed the Earth to be flat. A Aristotle 9Jc Try these… They believed the Earth to be flat. Thought that the Earth did not move. How the planets move around the Sun. Suggested the modern model of the Solar System. A Aristotle B Kepler C Egyptians D Ellipse Pearson Education Limited 2004

You should know… Why planets stay in their orbits. 9Jd You should know… Why planets stay in their orbits. What a satellite is and some uses of artificial satellites. About different kinds of orbits for artificial satellites, and why they are useful. How satellites can help us to explore the Solar System. Pearson Education Limited 2004

Gravity and the planets 9Jd Gravity and the planets The Sun’s gravity keeps the Earth and the other planets moving around it in elliptical orbits. keeps the Moon in its orbit. The Earth’s gravity The gravitational attraction between the stars in a galaxy holds the galaxy together. There is even an attraction between different galaxies. Pearson Education Limited 2004

Satellites Anything that orbits a planet is a satellite. 9Jd Satellites Anything that orbits a planet is a satellite. The Moon is an example of a natural satellite. Artificial satellites have been put into Earth’s orbit. They have many uses: communications satellites transmit TV pictures or phone calls the Global Positioning System (GPS) is a set of satellites that tells people their exact position on the ground. Pearson Education Limited 2004

scientific research and 9Jd Satellites Earth observation satellites take pictures or measurements of the Earth. They can be used for weather forecasting, making maps, scientific research and spying on other countries. Satellites can also be put into orbit around other planets. Space probes have been sent to Venus, Mars, Jupiter and Saturn. Pearson Education Limited 2004

9Jd Different orbits Communications satellites follow a geostationary orbit so they can be at the same point above the Earth at all times. They are about 36 000 km above the Earth and have a rotation period equal to the Earth’s rotation on its axis (24 hours). Their orbit is usually over the Equator. Pearson Education Limited 2004

9Jd Different orbits A satellite in a polar orbit covers all parts of the Earth as it spins. Observation satellites have polar orbits. path of satellite on its last orbit path of satellite this orbit path of satellite on its next orbit Pearson Education Limited 2004

9Jd Try these… Question: What type of orbit does the Earth have around the Sun? or What type of orbit is oval? Answer: Elliptical Pearson Education Limited 2004

What keeps the Earth from flying off into space? or 9Jd Question: What keeps the Earth from flying off into space? or What force exists everywhere in the Universe? What holds galaxies together? Answer: Gravity Pearson Education Limited 2004

What do we call anything that orbits a planet? or What is the Moon? 9Jd Question: What do we call anything that orbits a planet? or What is the Moon? Answer: A satellite Pearson Education Limited 2004

Which satellites can be used for weather forecasting? or 9Jd Question: Which satellites can be used for weather forecasting? or Which satellites follow polar orbits? Answer: Earth observation satellites Pearson Education Limited 2004

How much do you know? The force which tries to pull objects together. Gravity Pearson Education Limited 2004

How much do you know? Weight is measured in these units. N _ _ _ _ _ _ 9J How much do you know? Weight is measured in these units. N _ _ _ _ _ _ Newtons Pearson Education Limited 2004

How much do you know? Its gravity is about one-sixth of the Earth’s. 9J How much do you know? Its gravity is about one-sixth of the Earth’s. M _ _ _ Moon Pearson Education Limited 2004

9J How much do you know? A Greek philosopher who thought the planets and the Sun circled the Earth. A _ _ _ _ _ _ _ _ Aristotle Pearson Education Limited 2004

9J How much do you know? He first suggested our modern model of the Solar System. K _ _ _ _ _ Kepler Pearson Education Limited 2004

9J How much do you know? The type of orbit followed by the planets around the Sun. E _ _ _ _ _ _ _ _ _ Elliptical Pearson Education Limited 2004

How much do you know? The Moon is an example. 9J How much do you know? The Moon is an example. N _ _ _ _ _ _ s _ _ _ _ _ _ _ _ Natural satellite Pearson Education Limited 2004

9J How much do you know? Orbit that allows a satellite to remain in the same place above the Earth. G _ _ _ _ _ _ _ _ _ _ _ _ Geostationary Pearson Education Limited 2004