Work and Energy
Ch. 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 nonmechanical 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 has
the ability to do work.
Potential Energy
• Potential Energy is stored
energy resulting from the
relative positions of objects in a
system.
▫ Gravitational Potential Energy Energy associated with position of
object relative to Earth or
gravitational source.
• Two types of potential energy
are:
▫ Elastic Potential Energy –
energy stored in any type of
stretched or compressed elastic
material.
Ex:
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
Examples:
• 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.
• 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
Energy
Nonmechanical
Mechanical
Kinetic
Potential
Chemical
Nuclear
Thermal
Electrical
Section 9.3 Summary
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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.