15.1 Energy and Its Forms
• Avalanches have a great potential for danger,
so scientists research when to cause them in
order to keep skiers safe.
• Energy is the ability to do work.
• Energy causes change in
– Motion
– Sound
– Appearance
• Work is a transfer of energy.
• For example, marathon runners have a “carbload,” or eat pasta the night before the race to
be able to run 26.2 miles.
• It does not take energy to sit (0 J of work
done).
Kinetic Energy
• Kinetic energy is the energy of motion.
• Kinetos means “moving” in Greek.
• The kinetic energy of any moving object
depends upon its mass and speed.
• KE = ½ mv2
• KE = kinetic energy (J)
• M = mass (kg)
• V = velocity (m/s)
Kinetic Energy Problems
• The mass of an object is 2 kg. Its velocity is 4
m/s. What is its KE?
• KE = ½ mv2
• KE = ½ (2kg)(4 m/s)2
• KE = ½(2)(16)
• KE = 1(16)
• KE = 16 J
KE Problems
• The KE of an object is 400 J. Its velocity is 10
m/s. What is its m?
• KE = ½ mv2
• 400 = ½ (m)(10 m/s)2
• 400 = ½(m)(100)
• 4 = 1/2(m)
• m = 8 kg
KE Problems
• The KE of an object is 81 J. Its mass is 2 kg.
What is its velocity?
• KE = ½ mv2
• 81 = ½ (2kg) (v)2
• 81 = (1) (v)2
• 811/2 = (v2)1/2
• V = + or – 9 m/s
Potential Energy
• Potential energy is energy that is stored as a
result of position or shape.
• Examples are a stretched string, a diver at the
end of the board, a car at the top of a roller
coaster, etc.
• Food is also potential energy (it is stored to be
used later) and changes its chemical shape.
Gravitational Potential Energy
• Gravitational potential energy is potential
energy that depends upon an object’s height.
• Someone in a roller coaster car at the top of
the first hill has no KE, but a large PE (high
height).
• An object’s gravitational potential energy
depends on its mass, its height, and the
acceleration due to gravity.
Potential Energy Problems
•
•
•
•
•
PE = mgh
PE is in J
M is in kg
G is 10 m/s2
H is in m
PE Problems
• A 2 kg object is placed atop a 4 m shelf. What
is its potential energy?
• PE = mgh
• PE = (2)(10)(4)
• PE = 80 J
PE Problems
• An object has 90 J of PE and is on a 3 m shelf.
What is the object’s mass?
• PE = mgh
• 90 = (m)(10)(3)
• 90 = 30 m
• M = 3 kg
PE Problems
• A 2 kg object has a potential energy of 50 J.
What is its height?
• PE = mgh
• 50 = (2)(10)(h)
• 50 = 20 h
• 2.5 m = h
Elastic Potential Energy
• The potential energy of an object that is
stretched or compressed is elastic potential
energy.
• This includes anything that springs, such as
stringed instruments (guitars, etc.) and
bungee cords.
• This also includes anything that bounces, such
as a basketball.
Forms of Energy
• The major forms of energy are mechanical
energy, thermal energy, chemical energy,
electrical energy, electromagnetic energy, and
nuclear energy.
• These can be broken down to KE and PE.
Mechanical Energy
• The energy associated with the motion and
position of everyday objects is mechanical
energy.
• This includes not only machines but also
people on the move.
• Some mechanical energies come from
chemical energy being transformed.
Thermal Energy
• The total potential and kinetic energy of all
the microscopic particles in an object make up
its thermal energy.
• High temperatures give off light (photons).
• Transfer is from hot to cold.
• Endothermic- takes in heat
• Exothermic- gives off heat
Chemical Energy
• The energy stored in chemical bonds is
chemical energy.
• Broken bonds do work.
• ATP becomes ADP.
• Combustion of gasoline moves cars.
Electrical Energy
• The energy associated with electric charges is
electrical energy.
• Electrical forces are used to do work.
• Examples are batteries and lightning.
• Battery potential is measured in volts (V).
Electromagnetic Energy
• A form of energy that travels through space in
the form of waves is electromagnetic energy.
• This includes solar radiation, visible light, and
x-rays.
• X-rays were given the name because scientists
don’t know what they are (x is a variable in
Algebra).
Nuclear Energy
• The energy stored in atomic nuclei is known as
nuclear energy.
• The strong and weak nuclear forces store an
extremely high PE.
• Fission releases energy by splitting the nuclei.
• Fusion releases energy when less massive
nuclei come together.
• The sun undergoes fusion of H into He.