Physical Science / Unit 8: Work, Energy and Machines
8.6 Conservation of Energy
Notes
The law of conservation of energy: energy cannot be created or destroyed, only changed from one form to another.
- A falling ball changes PE to KE. At any point, $PE + KE$ stays the same (ignoring friction).
- A pendulum: PE is greatest at the top of the swing and KE is greatest at the bottom.
- Friction changes some mechanical energy into thermal energy, but the total is the same.
- Example: A ball at the top has 100 J of PE. Just before landing it has 100 J of KE. Halfway down, it has about 50 J PE and 50 J KE.
Practice
A roller coaster has 8,000 J of potential energy at the top of a hill. Ignoring friction, what is its kinetic energy at the bottom?
All the PE is converted to KE.
At one moment a swinging pendulum bob has 30 J of KE and 20 J of PE. What is its total mechanical energy?
$30 + 20 = 50$ J.
A sliding block comes to a stop. Where did its kinetic energy go?
Energy is conserved; friction converts it to heat.
Ready for more? Take the Unit 8 test.