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Chapter 4
Energy
Energy
4-1: The Nature of Energy
• When something is able to change its
environment or itself, it has energy.
Energy
• energy – the ability to cause change or the
ability to do work
• Types of Energy
• kinetic energy – energy in the form of motion
– the amount of kinetic energy that an object
has depends on its mass and velocity
– the greater the mass and the greater the
velocity the more its energy
– kinetic energy is measure in Joule (J)
Kinetic Energy
Energy
• energy – the ability to cause change or the
ability to do work
– equation: m = mass &
v = velocity
1
2
KE = m v
2
Energy
1
2
KE = m v
2
• If a truck is moving at a velocity of 30 m/s and
has a mass of 500 kg, what is its kinetic
energy?
Energy
• energy – the ability to cause change or the
ability to do work
• Types of Energy
• potential energy – energy that is stored and based
on an objects position
– potential energy can be converted to kinetic energy
when something acts to release it
• example: a book sitting on a shelf has potential
energy but when it falls that potential energy is
converted into kinetic energy
Energy
• Potential vs. Kinetic Energy
Energy
• potential energy – energy that is stored and
based on an objects position
• Types of Potential Energy
– elastic potential energy is stored by
something that can stretch or compress
• examples: rubber band, spring, or
shocks
Energy
• potential energy – energy that is stored and
based on an objects position
• Types of Potential Energy
– chemical potential energy is energy that is
stored in chemical bonds
• examples: food that we eat, gasoline
Energy
• potential energy – energy that is stored and
based on an objects position
• Types of Potential Energy
– gravitational potential energy (GPE) is
energy that is stored by objects that are above
the Earth’s surface
• examples: an apple falling from a tree, a
skydiver jumping out of an airplane
Energy
• gravitational potential energy (GPE) is
energy that is stored by objects that are above
the Earth’s surface
GPE = m g h
– m = mass (kg)
– g = acceleration due to gravity (9.8 m/s2)
– h = height (m)
Energy
GPE = m g h
• A 2 kg ball is dropped on Mr. Thomsen’s head from
a height of 10 m. Before it was dropped, how much
gravitational potential energy did the ball have?
Energy
• Relationship Between Kinetic & Potential
Energy
– As an object drops, potential energy
decreases but kinetic energy increases
– Since an object is moving while it is
dropping, it is gaining kinetic energy
– Since an object is no longer at a higher
elevation, it is losing potential energy
Energy
• Potential vs. Kinetic Energy
Conservation of Energy
• Energy is constantly being transformed from
one form to another. For example, a light bulb
is transforming electrical energy into light
energy, or a car engine transforms chemical
energy stored in gasoline into kinetic energy.
Conservation of Energy
• mechanical energy – the total amount of
potential and kinetic energy in a system
• Mechanical Energy
= Potential Energy + Kinetic Energy
– involves both position and motion of an object
– the total mechanical energy stays constant
while an object falls because potential energy
decreases but kinetic energy increases
Conservation of Energy
• Mechanical Energy
Conservation of Energy
• Law of Conservation of Energy – energy
cannot be created or destroyed only
transferred from one form to another
– this law implies that the amount of energy in
the universe remains constant
Conservation of Energy
• Law of Conservation of Energy – energy
cannot be created or destroyed only
transferred from one form to another
– example: If you are on a swing and stop
pumping your legs, you will eventually come
to a stop. So, the swing will then have NO
kinetic energy. Where does the energy go?
• The swing’s ropes rub on their hooks and
some of the energy turns into thermal energy
(heat). Air resistance can cause an increase
in air temperature.
Conservation of Energy
• Law of Conservation of Energy – energy
cannot be created or destroyed only
transferred from one form to another
– The swing’s ropes rub on their hooks and
some of the energy turns into thermal energy
(heat). Air resistance can cause an increase
in air temperature.
– Energy Lost = Energy Gained