What is kinetic energy?

2. 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?

3. KE = ½ x mass x velocity2 = ½mv2 How is kinetic energy calculated? The kinetic energy (KE) of an object can be calculated using this equation: • Mass is measured in kilograms (kg). • Velocity is measured in meters per second (m/s). • KE is measured in joules (j).

4. Calculating kinetic energy question A truck with a mass of 1,500kg travels at a velocity of 20m/s. What is the kinetic energy of the truck? kinetic energy = ½ xmass x velocity2 = ½ x 1,500 x 202 = ½ x 1,500 x 400 = 750 x 400 = 300,000J = 300kJ

5. KE, mass and velocity calculations

6. The kinetic energy of cars

7. 1,000kg 2,000kg 20mph 40mph 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. KE = 40kJ KE = 80kJ KE = 160kJ KE = 320kJ What factor – mass or velocity – has the greatest effect on the kinetic energy of a moving object?

8. 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.

10. A long way down… How would you describe a bungee jumper? As someone who is: • brave • insane • 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.

11. 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.

12. GPE = mass x gravitational acceleration x height How is GPE calculated? The GPE of an object can be calculated using this equation: • Mass is measured in kilograms (kg). • Gravitational acceleration is the same for all objects (ignoring air friction) and is equal to 9.8m/s2. (For our calculations we can round up and just use “10”.) • Height is measured in meters (m). • GPE is measured in joules (j).

13. Calculating GPE question 1 An eagle with a mass of2kg flies at a height of200m 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 = 4,000J

14. Factors affecting GPE

15. GPE, mass and height calculations

17. Energy cannot be created or destroyed, just changed in form. 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: 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.

18. Gerald the Human Cannonball

19. GPE lost = KE gained 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. 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 a rollercoaster would be less than the GPE lost.

20. Energy transfer of rollercoasters

22. What is a useful energy transfer? electrical energy kinetic energy Many household objects are designed to transfer energy from one form into another useful form. What energy transfer is an electric fan designed to carry out?

23. What is a useful energy transfer? electrical energy sound energy What energy transfer are these speakers designed to carry out?

24. What is a useful energy transfer? electrical energy kinetic energy What energy transfer are wind turbines in a wind farm designed to carry out?

25. What is a useful energy transfer? gravitational potential energy electrical energy What energy transfer is a hydroelectric power station designed to carry out?

26. Energy transfer in a television An energy transfer diagram shows the input and output energies for a device. This includes all the useful and wasted forms of energy. For example, in a television: light electrical sound heat

27. Energy transfer in a car engine What are the main energy transfers in a car engine? (Don’t forget the wasted energy.) kinetic chemical sound heat

28. How can we represent energy transfers? All the energy transfers (useful and wasted) that are associated with a device can be represented by a Sankey diagram. Filament light bulb 100J electrical energy (input) 10J light energy (output) A Sankey diagram uses arrows to represent all the output energies. 90J heat energy (wasted) The thickness of each arrow is proportional to the amount of energy involved at that stage. Energy efficient light bulb 20J electrical energy (input) 10J light energy (output) 10J heat energy (wasted) How does the energy use in these light bulbs compare?

29. Useful energy transfers