Work and Energy

1. Work and Energy Ch. 9.3 What is Energy?

2. Section 9.3 Objectives • Explain the relationship between energy and work. • Define potential energy and kinetic energy. • Calculate kinetic energy and gravitational potential energy. • Distinguish between mechanical and non-mechanical energy. • Identify non-mechanical forms of energy.

3. Energy and Work • Energy is the ability to do work. • Both Work and Energy are measured in Joules (J). A stretched slingshot has the ability to do work.

4. Potential Energy • Potential Energy is stored energy resulting from the relative positions of objects in a system. • Two types of potential energy are: • Elastic Potential Energy – energy stored in any type of stretched or compressed elastic material. Ex: • Gravitational Potential Energy -Energy associated with position of object relative to Earth or gravitational source. Formula : PEg = Mass x Free-fall acceleration x height PEg = mgh Where m = mass in kilograms (kg) g = gravity 9.8 m/s2 h = height in meters (m)

5. Kinetic Energy • Kinetic energy is the energy of a moving object due to its motion. • Kinetic energy depends on both mass and speed of an object. • Formula : kinetic energy = ½ x mass x speed squared KE = ½ mv2 • Notice speed is squared; KE depends on speed more than mass.

6. Mechanical and Non-mechanical Energy • Together, the sum of potential energy and kinetic energy make up mechanical energy. • Energy that lies at the level of atoms and that does not affect motion on a large scale is known as non-mechanical energy. Examples: • Atoms & molecules – kinetic energy • Chemical reactions – potential energy • Photosynthesis – chemical energy • Food – chemical energy • Sunlight – solar energy • Electricity – electric energy • Electromagnetic (light) waves – non mechanical

7. Forms of Energy

8. Section 9.3 Summary • Energy is the ability to do work. • Like work, energy is measured in joules. • Potential energy is stored energy. • Elastic potential energy is stored in any stretched or compressed elastic material. • The gravitational potential energy of an object is determined by its mass, its height, and g, the free-fall acceleration due to gravity. PE = mgh. • An object’s kinetic energy is energy of motion, is determined by its mass and speed. KE = ½ mv2. • Potential energy and kinetic energy are forms of mechanical energy. • In addition to mechanical energy, most systems contain non mechanical energy. • Non mechanical energy does not usually affect systems on a large scale.