Lesson 7 Weight Mass and Gravity Gravity and

Lesson 7 Weight, Mass and Gravity

Gravity and Newton Gravity is the most important force in the universe! It is an attractive force that makes apples fall from trees and the planets orbit the Sun. Sir Isaac Newton was a scientist who was interested in forces and gravity. He did not discover gravity – gravity and its effects existed long before Newton did discover how to calculate the force of gravity on objects. His work was so important that the unit of force is named after him! 1 20 2 of 27 © Boardworks Ltd 2006 2004

Gravity is an attractive force that affects anything with mass Even people attract other objects with gravity but our mass isn’t large enough for the force to be very strong Isaac Newton discovered gravity when an apple fell on his head

Gravity and mass Gravity is an attractive force that acts between all objects that have mass. The size of the force depends on the mass of the object. All objects produce a gravitational force. This is a massive force for huge masses such as a planet. Think about it: When you jump, the gravitational force of the Earth pulls you down. Your gravitational force is also pulling the Earth towards you – you just don’t notice this effect because your mass is so much smaller than Earth’s! 1 20 4 of 27 © Boardworks Ltd 2006 2004

Mass is the amount of matter (or stuff) an object contains Mass is measured in grams (g) An objects mass is the same wherever it is An Elephant contains more matter than a mouse so has more mass

Gravity pulls down to the centre of the earth no matter where you stand on it GRAVITY

Gravity and distance The force of gravity between two objects depends on the mass of the objects and also the distance between them. Spacecraft produce a very large force, called thrust, to overcome the force of gravity As a spacecraft gets further away from Earth, the force of gravity gets smaller. Why do spacecraft lose their large fuel tanks and booster rockets once they have left the Earth’s surface? 1 20 7 of 27 © Boardworks Ltd 2006 2004

Distance and Gravity gets weaker the further away you get from an object Every time you double the distance from an object the effect weakens by four times You don’t have to be far from earth to appear weightless

What are mass and weight? Mass and weight are not the same! Mass is the amount of matter in an object and is measured in kilograms. Mass is not a force. Mass will have the same value anywhere in the Universe, including outer space. Weight is a force and is caused by the pull of gravity acting on a mass. Like other forces, weight is measured in newtons and has both magnitude and direction. Weight has different values depending on where you are in the Universe. 1 20 9 of 27 © Boardworks Ltd 2006 2004

Weight and mass on the Moon The force of gravity is less on the Moon than it is on Earth. This is because the Moon has a smaller mass than the Earth. Any object will weigh less on the Moon than it does on Earth. An astronaut could jump 20 feet into the air on the Moon because gravity is less. However, the astronaut still has the same body and the same mass: he just weighs less because gravity is weaker on the Moon. 1 10 ofof 20 27 © Boardworks Ltd 2006 2004

Talking about weight and mass A scientist should never say: “She weighs 50 kilograms, ” but should always say: “She has a mass of 50 kilograms”. Alternatively, the scientist could say: “The gravitational force acting on her mass is about 500 newtons”. This is the same as saying: “Her weight is about 500 newtons”. 1 11 ofof 20 27 © Boardworks Ltd 2006 2004

Mass and Gravity The effects of gravity only become significant when objects get to the mass of a moon or planet The more mass an object has the stronger its gravity will be The sun has the most mass out of anything in our solar system so it has the most gravity

Mass and Gravity and Mass are directly proportional This means as you increase one the other increases at the same rate If you double the mass of an object you double its gravity

F = Force of Gravity F F/4 F/16

Gravity Summary Gravity is changed by two things: • Mass: When we double the mass we double the effects of gravity • Distance: When we double the distance we divide the effects of gravity by four

Orbits Orbiting is when an object travels around another due to gravity Objects will only orbit things with a higher gravitational force than themselves E. g. The Moon orbits the Earth and the Earth orbits the Sun Orbits are usually circular but not always They can also be elliptical

Planet Orbits The further away a planet is from the sun the longer it takes for it to orbit it The time for a planet to orbit a star is called a year A year on Earth is longer than a year on Mercury The length of a year on Earth is 365 days On Neptune a year is 60048 days long

Planet Orbits The further away from the Sun a planet is the slower it travels

Weight is a force caused by gravity Weight is the gravitational force between objects Because weight is a force it is measured in Newtons (N) People usually use the word weight wrong and get it confused with mass

Weight, Mass and Gravity Wherever you are in the universe your mass will be the same But depending on how strong the gravity is you can weigh different amounts Because gravity is weaker on the moon astronauts weigh very little

Mass 300 Kg Weight 3000 N On earth you wouldn’t be able to lift a safe because it would weigh too much

Mass 300 Kg Weight 500 N On the moon the safe would have the same mass but its weight would be lower

Mass 300 Kg Weight 500 N Because it weighs less you would be able to pick up the safe

Gravity Calculations Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg) Earths gravitational field strength is 10 N/Kg If a man had a mass of 80 kg then you could enter those numbers into the formula to calculate his weight Weight = 10 x 80 = 800 N W M x G W: Weight M: Mass G: Gravitational Field Strength

1 25 ofof 20 27 © Boardworks Ltd 2006 2004

1 26 ofof 20 27 © Boardworks Ltd 2006 2004

Weight and mass activity 1 27 ofof 20 27 © Boardworks Ltd 2006 2004

Missing words 1 28 ofof 20 27 © Boardworks Ltd 2006 2004

Questions