Download 5.11 Potential and Kinetic Energy

Do Now: What is
energy? What are
the different
types of energy?
Test Make-Ups?
Starting the last part of
physics Today!
What is energy?
Energy is the ability to do work
The more energy something has, the more work it can do.
Remember…
Work = Force x Distance
or
w=f x d
Video Clip
Potential and Kinetic Energy
Kinetic Energy: Energy of Motion
Potential Energy: Energy of Position
Kinetic Energy Examples
The moving arrow has kinetic
energy.
The falling water has kinetic energy.
Potential Energy Examples
The archer pulled back on the
arrow which now has potential
energy.
The water at the top of a waterfall
has gravitational potential energy
(stored energy due to its height).
Elastic potential energy- occurs when an
object is stretched or compressed
Examples: bow, rubber band, spring
Identifying Potential and Kinetic Energy
Identifying Potential and Kinetic Energy
Kinetic Energy Calculations
Kinetic Energy = ½ mass x velocity2
KE= ½ m x v2 or KE = m x v2
2
Units: Joules (1 J = 1 N•m)
Example: What is the kinetic energy of a 24 kg ball
that is moving with a velocity of 2 m/s?
KE= m x v2
2
KE= 24 kg x (2 m/s) 2
2
KE= 48 Joules
Kinetic Energy Calculations
Example: A 70 kg boulder is rolling down a hill
towards you at a velocity of 5 m/s? What is its
kinetic energy?
KE= m x v2
2
KE= 70 kg x (5 m/s) 2
2
KE= 875 Joules
Potential Energy Calculations
Potential Energy= weight x height
PE= weight x height
Units: Joules
But…how do we calculate weight?!?!?!
Weight= mass x acceleration of gravity (9.8 m/s2)
weight= m x agravity
agravity=9.8 m/s2 ALWAYS!!
Units: kg-m or Newtons
s2
Mass vs. Weight
Mass is the amount of matter an object
contains. An object’s mass is the same
everywhere in the universe.
The unit is grams or kilograms.
Weight is the force of gravity on an object.
Your weight on earth is different from your
weight on the moon, on Phobos, etc. The
unit is Newtons.
Potential Energy Calculations
Example: What is the potential energy of a 10 kg box
being lifted to a height of 2 meters?
First…What is the weight of the box?
weight= m x agravity
agravity=9.8 m/s
Now we can calculate the PE…
PE = weight x height
PE = 98 N x 2 m
PE = 196 Joules
weight= 10 kg x 9.8 m/s
weight = 98 kg m = 98 N
s
Potential Energy Calculations
Example: A rock with a weight of 156 N falls 5 m. What
potential energy does it have just before the end of its
fall?
Sweet! We already have the weight!
Now to calculate the PE…
PE = weight x height
PE = 156 N x 5 m
PE = 780 Joules
Brain Pop: KE
Brian Pop: PE