9.3 – What is Energy?
Energy and Work – when work is done, energy is transformed or transferred.
Energy is the ability to do work.
 Energy is measured in joules (the same unit used for work)
Potential Energy
 Potential energy is stored energy. Potential energy is sometimes called
energy of position because it results from the relative positions of objects in
a system.
o Elastic potential energy – energy stored in any type of stretched or
compressed elastic material (spring, rubber band)
o Gravitational potential energy – energy that could potentially do work
on an object resulting from its position above the ground
o Gravitational Energy Equation
 Depends on mass and height
Grav. PE = mass x acceleration due to gravity x height
PE = mgh
PE = wh
(same as the work equation: W = Fd)
o Calculate the gravitational potential energy of a 0.52 kg bird flying at
an altitude of 550 m.
Kinetic Energy – the energy an object has because of its motion
 Kinetic energy depends on mass and speed
o Kinetic Energy Equation
KE = ½ X mass X velocity squared
KE = ½ mv2
o Calculate the kinetic energy in joules of a 1500 kg car moving 18 m/s.
 Kinetic energy depends on speed more than mass (speed is more important)
Other Forms of Energy – the sum of potential energy and kinetic energy in a
system is called mechanical energy. Mechanical energy is generally thought to be
the potential and kinetic energy that affects motion on a large scale (you can see
the motion happening.)
 Thermal Energy – atoms and molecules have kinetic energy (related to an
object’s temperature)
 Chemical Energy – Chemical reactions involve potential energy. (The
amount of chemical energy associated with a substance depends in part on
the relative positions of the atoms it contains.)
 Light Energy – Living things get energy from the sun. (Photosynthesis turns
solar energy into chemical energy) Light can carry energy across empty
 Nuclear Energy – The sun gets energy from nuclear reactions. (Nuclear
energy is a kind of potential energy stored by the forces holding subatomic
particles together in the nuclei of atoms.)
 Electrical Energy – Electricity is a form of energy. (Electricity results from
the flow of charged particles through wires or other conducting materials.)