Download What is Energy? - CEC

Physics of Energy
What is Energy?
Energy is simply the ability to do work. Pretty much
everything you do each day requires some type of energy.
For example, you can’t run up stairs (work) without energy! It
takes a LOT of energy to climb the world’s longest staircase.
The Niesenbahn funicular staircase, located on Neisen
mountain in Switzerland, has 11,674 steps!
Whenever something
moves, you can see the
change in the energy
of that system. Energy
can make things move
or cause a change in
the position or state
of an object.
Main Types of Energy
•Kinetic Energy
•Gravitational potential energy (also called
just “potential energy”
•Thermal Energy (heat)
•Nuclear Energy (nuclear power, atomic bomb)
•Electrical Energy (electricity)
•Chemical Energy (food, fire, …)
First, we are going to take an in-depth look at
the first two types of energy.
Kinetic Energy
Kinetic energy is the
energy of motion
KE = ½ mv^2
Units: Joules (J)
m = mass
v = velocity
Let’s Practice
What is the kinetic energy of this 1,550-kg car
when it is traveling 38.0 m/s?
KE = ½ m v^2
= ½ (1550)(38.0)^2
= 1119100 J
= 1.11 x 10^6 J
Potential Energy
Gravitational Potential Energy
(also referred to as just
“potential energy”) is energy due
to an objects height above the
ground
PE = mgh
m = mass
g = acceleration due to gravity
h = height
Let’s Practice
A rock has a mass of 8.40 x 10^4 kg.
The center of mass is 29.0 m above
the ground. How much energy does it
have?
PE = mgh
Identify the variables:
m= 8.40 x 10^4 kg
g= 9.8 m/s^2
h= 29.0 m
PE = (8.4X10^4)(9.8)(29)
= 2.39 x 10^7 J
What’s so important about PE and KE?
We call the sum of PE and KE
mechanical energy:
ME = KE + PE
Mechanical energy is important because it is
conserved (as long as there are no non conservative
forces, like friction)
Therefore, if one goes down, the other goes up by the
same amount. We can use this when trying to create
machines/technology that helps to provide an
efficient means of energy.
Conceptual Understanding
A penny is dropped off the Eiffel tower (ignore air
resistance).
As it falls, what happens to its potential energy?
What happens to its kinetic energy?
As it falls, its velocity goes up, so
its kinetic energy goes up. It also
loses height so its potential
energy goes down.
However, mechanical energy stays
the same ME = KE + PE (no “new”
energy is created or destroyed!)
Let’s Practice
KE = ½ m v^2
KE = ½ (2) (3)^2
KE = (1) (9)
KE = 9.0 J
Let’s Practice
PE = mgh
m= 50 kg
g= 9.8 m/s^2
h= 10 m
PE = (50)(9.81)(10)
= 4,900 J (1/2 of that = 2,450 J)
Let’s Practice