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1 of 8 © Boardworks Ltd 2008 Warm Up 9/24 Directions: Write which of Newton’s Laws these sentences describe: 1. I can move the small snowball I made for the snowman’s head, but I will need help moving his belly. 2. When I push back with my ski poles, I move forward. 3. The harder I pull, the faster my dog runs. 2 of 8 © Boardworks Ltd 2008 Potential Energy • Potential Energy is stored energy. • Ex. A stretched rubber band has the potential or ability to fly across the room. 3 of 8 © Boardworks Ltd 2008 Types of Potential Energy 1. Gravitational Potential Energy 2. Chemical: This type of energy is stored when you eat food or burn an object 3. Electrical: Used to power houses (also found in batteries) 4. Elastic: Stored when you stretch a rubber band (earthquakes too) 5. Nuclear: Produced by splitting the nucleus of an atom. 4 of 8 © Boardworks Ltd 2008 A long way down… How would you describe a high diver? As someone who is: brave insane full of beans full of gravitational potential energy (GPE). GPE is the amount of energy an object has because of its position above the ground, i.e. its height. 5 of 8 © Boardworks Ltd 2008 What is gravitational potential energy? The gravitational potential energy (GPE) of an object on Earth depends on its mass and its height above the Earth’s surface. When a bungee jumper starts to fall, he starts to lose GPE. As the elastic cord pulls the bungee jumper back up, he gains GPE. 6 of 8 © Boardworks Ltd 2008 How is GPE calculated? The GPE of an object can be calculated using this equation: GPE = mass x gravitational field strength x height Mass is measured in kilograms (kg). Gravitational field strength is measured in newtons per kilogram (N/kg), usually taken as 10 N/kg (9.8) on Earth. Height is measured in meters (m). GPE is measured in joules (j). 7 of 8 © Boardworks Ltd 2008 Factors affecting GPE 8 of 8 © Boardworks Ltd 2008 Calculating GPE question 1 An eagle with a mass of 2 kg flies at a height of 200 m above the ground. How much gravitational potential energy does the eagle have? GPE = mass x gravitational field strength x height = 2 x 10 x 200 = 4,000 J 9 of 8 © Boardworks Ltd 2008 Calculating GPE question 2 An apple with a mass of 200 g falls 3 m from its branch to the ground. How much GPE will the apple have lost when it reaches the ground? change GPE = mass x gravitational field strength x in height lost = 0.2 x 10 x 3 = 6J 10 of 8 © Boardworks Ltd 2008 GPE, mass and height calculations 11 of 8 © Boardworks Ltd 2008 12 of 11 © Boardworks Ltd 2008 What is kinetic energy? The word ‘kinetic’ comes from the Greek word ‘kinesis’, meaning motion. Kinetic energy is the energy an object has because it is moving. All moving things have kinetic energy, but the amount of energy they have is not just dependent on how fast they are moving. What other factors affect the kinetic energy of a moving object? 13 of 8 © Boardworks Ltd 2008 Kinetic Energy 14 of 8 © Boardworks Ltd 2008 Kinetic Energy 15 of 8 © Boardworks Ltd 2008 How is kinetic energy calculated? The kinetic energy (KE) of an object can be calculated using this equation: KE = ½ x mass x velocity2 = ½mv2 Mass is measured in kilograms (kg). Velocity is measured in meters per second (m/s). KE is measured in joules (j). 16 of 8 © Boardworks Ltd 2008 Calculating kinetic energy question A truck with a mass of 1,500 kg travels at a velocity of 20 m/s. What is the kinetic energy of the truck? kinetic energy = ½ x mass x velocity2 = ½ x 1,500 x 202 = 300,000 J = 300 kJ 17 of 8 © Boardworks Ltd 2008 Rearranging the KE equation Sometimes it is necessary to rearrange the kinetic energy equation in order to calculate the mass or the velocity of a moving object. KE = ½mv2 What are the rearranged versions of this equation for calculating mass and velocity? m = 18 of 8 2KE v2 v = 2KE m © Boardworks Ltd 2008 KE, mass and velocity calculations 19 of 8 © Boardworks Ltd 2008 Too much kinetic energy Doubling the mass of a moving object doubles its kinetic energy, but doubling the velocity quadruples its kinetic energy. If the velocity of a car is slightly above the speed limit, its kinetic energy is much greater than it would be at the speed limit. This means that: It is more difficult to stop the car and there is more chance of an accident. It the car does collide with something, more energy will be transferred, causing more damage. 20 of 8 © Boardworks Ltd 2008 The kinetic energy of cars 21 of 8 © Boardworks Ltd 2008 Dangerous speeding? Use the KE = ½mv2 equation to fill in the kinetic energy values in the table below for two cars each traveling at two different velocities. 1,000 kg 2,000 kg 20 mph KE = 40 kJ KE = 80 kJ 40 mph KE = 160 kJ KE = 320 kJ What factor – mass or velocity – has the greatest effect on the kinetic energy of a moving object? 22 of 8 © Boardworks Ltd 2008 Feeling energetic? Who has the most kinetic energy: Rita or the cat? 23 of 8 © Boardworks Ltd 2008 Calculating velocity question A truck has a mass of 20,000 kg. If its kinetic energy is 2.25 mJ, at what velocity is it travelling? KE = ½ x mass x velocity2 velocity = = 2KE mass 2 x 2,250,000 20,000 = 15 m/s 24 of 8 © Boardworks Ltd 2008