Matter & Energy #2

1. Matter & Energy #2 Kinetic & Potential Energy Energy Sources & Alternatives

2. Energy • Energy- Energy is the ability to do work, and to do work we need energy. • Ex. A stretched slingshot has the ability to do work • The units for energy is the Joule (J). • Energy can be present in an object or a system when nothing is happening at all. (see ex above).

3. Energy of Motion • Potential energy- stored energy resulting from the relative position of objects in a system.

4. Types of Potential Energy • There are two (2) types of PE. • Elastic PE: the energy stored in any type of stretched or compressed elastic material, such as clock spring or a bungee cord.

5. Types of Potential Energy cont. • Gravitational PE: The energy stored in an object due to its position above the ground. • Any system of two or more objects separated by distance contains gravitational PE.

6. Gravitational Potential Energy Equation • Gravitational PE depends on both mass and height. • Gravitational PE Equation: • gPE = mass X free fall acceleration X height PE=mgh • Note: m needs to be in kilograms (kg), h needs to be in meters (m) and g = 9.81m/s2, ALWAYS. • If we rearrange the PE formula and solve for h, we get: h= PE/mg • To solve for m, we get: m= PE/gh

7. gPE Practice Problems • PE=m X g X h • A 65 kg rock climber ascends a cliff. What is the climber’s gPE at 35 m above the base of the cliff? • A science student holds a 55g egg out a window. Just before the egg is dropped it has 8J of gPE with respect to the ground. How far is the student’s arm from the ground. • A diver has 3400J of gPE after stepping up onto a diving platform that is 6.0 m above H2O. What is the diver’s mass?

8. Kinetic Energy • KE is defined as energy of motion. • This is obtained when the PE is released. Therefore PE changes to KE.

10. Kinetic Energy Equation • KE depends on mass and speed. • KE Equation: • KE= ½ X mass X speed squared • ONLY SPEED (v) is squared!!! • KE depends on speed more than mass.

11. KE Practice Problems • What is the kinetic energy of a 44 kg cheetah running at 31 m/s? • A 35 kg child has 190 J of kinetic energy after sledding down a hill. What is the child’s speed in meters per second at the bottom of the hill? • A bowling ball traveling 2.0 m/s has a 16 J of kinetic energy. What is the mass of the bowling ball in kilograms?

12. Other Forms of Energy • Mechanical energy: The sum of the PE and KE in a system. This type of energy affects the motion of the object or thing in question. This sum is simply referred to as the total mechanical energy (abbreviated TME). TME = PE + KE

13. The TME is = to 50,000 J at every position.

14. Other Forms of Energy cont. • Non-mechanical energy: this is energy that lies at the level of atoms and does not affect motion on the large scale. • Note: Other forms of energy are just special cases of KE or PE. • EX: If you eat food, you will receive energy from it.

15. Other Forms of Energy cont. • Atoms and molecules have KE. • Atoms and molecules are always in constant motion. • As the temp increase their motion increases, and as the temp decreases their motion decreases.

16. Other Forms of Energy cont. • Chemical Energy: a type of PE. • When a chemical rxn occurs, bonds are broken and new ones form. • This formation and breaking of bonds involve changes in energy because chemical energy depends on position of the bonds.

17. Other Forms of Energy cont. • Living things get energy from the sun. • Where do you get the energy you need to live? • It comes in the form of chemical energy stored in food. • Plants get their energy from the sun through a process called photosynthesis. We, in turn, obtain energy through a process called cellular respiration.

18. Photosynthesis

19. Cellular Respiration

20. Other Forms of Energy cont. • The sun gets energy from nuclear fusion. • Nuclear fusion- a rxn in which light atomic nuclei combine to form a heavier nucleus. In this process a enormous amount of energy is released.

21. Other Forms of Energy cont. • Electricity is a form of energy. • Light can carry energy across empty space.