Work and EnergyCh. 9.3 What is Energy?

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.

Energy and Work

•Energy is the ability to do work.

•Both Work and Energy are measured in Joules (J).

A stretched slingshot hasthe ability to do work.

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)

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.

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

Forms of Energy

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.