1 of 30 © Boardworks Ltd 2007 2 of 30 © Boardworks Ltd 2007 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. 3 of 30 © Boardworks Ltd 2007 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. GPE is the amount of energy an object has because of its position above the ground, i.e. its height. 4 of 30 © Boardworks Ltd 2007 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 on Earth. Height is measured in metres (m). GPE is measured in joules (j). 5 of 30 © Boardworks Ltd 2007 Factors affecting GPE 6 of 30 © Boardworks Ltd 2007 Calculating GPE question 1 An osprey with a mass of 2 kg flies at a height of 200 m above the ground. How much gravitational potential energy does the osprey have? GPE = mass x gravitational field strength x height = 2 x 10 x 200 = 4,000 J 7 of 30 © Boardworks Ltd 2007 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 8 of 30 © Boardworks Ltd 2007 GPE, mass and height calculations 9 of 30 © Boardworks Ltd 2007 10 of 30 © Boardworks Ltd 2007 Feeling energetic? Who has the most kinetic energy: Rita or the cat? 11 of 30 © Boardworks Ltd 2007 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? 12 of 30 © Boardworks Ltd 2007 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 metres per second (m/s). KE is measured in joules (j). 13 of 30 © Boardworks Ltd 2007 Calculating kinetic energy question A car with a mass of 1,500 kg travels at a velocity of 20 m/s. What is the kinetic energy of the car? kinetic energy = ½ x mass x velocity2 = ½ x 1,500 x 202 = 300,000 J = 300 kJ 14 of 30 © Boardworks Ltd 2007 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 = 15 of 30 2KE v2 v = 2KE m © Boardworks Ltd 2007 Calculating velocity question A lorry 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 16 of 30 © Boardworks Ltd 2007 KE, mass and velocity calculations 17 of 30 © Boardworks Ltd 2007 The kinetic energy of cars 18 of 30 © Boardworks Ltd 2007 Dangerous speeding? Use the KE = ½mv2 equation to fill in the kinetic energy values in the table below for two cars each travelling 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? 19 of 30 © Boardworks Ltd 2007 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 30 © Boardworks Ltd 2007 21 of 30 © Boardworks Ltd 2007 What is conservation of energy? There are many different forms of energy, such as kinetic, sound, thermal and light energy. Each form of energy can be transferred or converted into an another form. All energy transfers follow the law of conservation of energy: Energy cannot be created or destroyed, just changed in form. This means that energy never just ‘disappears’. The total amount of energy always stays the same, i.e. total input energy = total output energy. In most energy transfers, the energy is transferred to several different forms, which may or may not be useful. 22 of 30 © Boardworks Ltd 2007 Gerald the Human Cannonball 23 of 30 © Boardworks Ltd 2007 Energy transfer of falling objects What happens to the KE and GPE of a rollercoaster? 24 of 30 © Boardworks Ltd 2007 The relationship between GPE and KE The law of conservation of energy means that as an object falls, the GPE it loses must turn into a different form. GPE lost = KE gained This is only true if air resistance and friction are ignored. In reality, GPE would also be transferred into heat and sound energy so the KE of rollercoaster would be less than the GPE lost. 25 of 30 © Boardworks Ltd 2007 Energy transfer of rollercoasters 26 of 30 © Boardworks Ltd 2007 Terminal Velocity 27 of 30 © Boardworks Ltd 2007 Identifying forces 28 of 30 © Boardworks Ltd 2007 What are resultant forces? There are usually several different forces acting on an object. The overall motion of the object will depend on the size and direction of all the forces. The motion of the object will depend on the resultant force. This is calculated by adding all the forces together, taking their direction into account. 50 N 30 N Resultant force on the crate = 50 N – 30 N = 20 N to the left 29 of 30 © Boardworks Ltd 2007 Calculating resultant forces 30 of 30 © Boardworks Ltd 2007 What is friction? Friction is a resistive force that slows things down and tries to stop objects sliding past each other. friction pulling force Friction always acts in the opposite direction to which an object is moving or trying to move. What would happen if friction didn’t exist? 31 of 30 © Boardworks Ltd 2007 Different types of friction Friction is most obvious when it acts between two solid objects, but it also acts between solid objects and gases, and between solid objects and liquids. Friction caused by an object moving through air is called air resistance. Friction caused by an object moving through a liquid, such as water, is called drag. 32 of 30 © Boardworks Ltd 2007 Introducing balanced forces 33 of 30 © Boardworks Ltd 2007 Terminal velocity of a skydiver 34 of 30 © Boardworks Ltd 2007 Velocity–time graph of skydiver 35 of 30 © Boardworks Ltd 2007 36 of 30 © Boardworks Ltd 2007 Glossary conservation of energy – The law that states that energy cannot be created or destroyed, just transferred into different forms. energy transfer – A process in which one form of energy is transferred into another form. gravitational field strength – The strength of gravity in a particular location. gravitational potential – The energy an object has because of its position in a gravitational field. joule – The unit of energy. kinetic – The energy an object has because it is moving. velocity – A measure of the speed and direction of a moving object. 37 of 30 © Boardworks Ltd 2007 Anagrams 38 of 30 © Boardworks Ltd 2007 Multiple-choice quiz 39 of 30 © Boardworks Ltd 2007