Download KE and PE - Fort Bend ISD

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 When
a woman stands with two feet on a
scale, the scale reads 400 N. When she
lifts one foot, what does the scale read
and why?
1
Energy
Energy
 the
ability to do work.
 Unit is Joules.
 There are two types of Energy that
we will be talking about:


Kinetic Energy – The energy in motion
Potential Energy – stored energy
Kinetic Energy
 Kinetic
energy (KE ) - the energy
an object possesses due to its
motion.
1 2
KE  mv
2
Example 1
Potential Energy
 When
work is done a resistive force,
potential energy (PE) is created
equal in magnitude to the work done.
PE = mgh
 Potential
energy is sometimes called
stored kinetic energy.
Potential Energy



Potential energy is stored energy.
 Anytime a particle or object is forced to maintain
a position in which it would not naturally exist has
potential energy.
 This natural position is known as equilibrium.
Types of Potential Energy (mechanical)
 Gravitational Potential Energy
 Elastic Potential Energy
Types of Potential Energy (non-mechanical)
 Electrical Potential Energy
 Chemical Potential Energy
Example 2
Mechanical Energy
 You
can have an energy system with
both KE and PE present at the same
time. (ignore friction)

Ex:Pendulum
• A pendulum system shows the interaction of
kinetic and potential energy
Pendulum at
Rest: No kinetic
and no potential
energy
Pull the
pendulum up:
at its highest
point, it has all
potential
energy and no
kinetic energy
As it moves through
its arc the
pendulum’s PE is
converted to KE
Both PE and KE are
present at the same
time.
At bottom of
the pendulum’s
arc all PE is
converted to
KE and the
pendulum is
moving at its
highest
velocity
KE provides
energy for the
pendulum to do
work and push the
pendulum upward
against gravity
KE provides
energy for the
pendulum to do
work and push the
pendulum upward
against gravity
At the top of the
arc, all KE is
converted to PE
16
Mechanical Energy
 Without
considering other energies (thermal
and chemical energies are negligible),
 ME = KE + PE
 Energy is conserved
Conservation Law
 In
the absence of friction or work,
the total mechanical energy
remains the same. This is
statement is called the Law of
Conservation of Mechanical
Energy.
KEi  PEi  KE f  PE f
Transformation from GPE to KE of a
Falling Object
100% GPE
0 % KE
75 % GPE
25% KE
50 % GPE
50% KE
25% GPE
75% KE
0% GPE
100% KE
Example 3
What is the velocity of a dropped 4.5 kg
bowling ball the instant it hits the ground if
it is dropped from a height of 13.6 m?
Example 4
A diver dives off a platform 50 m above a
tank of water. If he has a mass of 56 kg
what will be his kinetic energy, potential
energy and speed
a) 10 m below the top of the platform
b) 30 m below the top, and
c) the instant he hits the water?
Example 5

An 8.0 kg flower pot falls from a window ledge 12.0 m above a
sidewalk. (a) What is the kinetic energy of the pot just as it reaches
the sidewalk? (b) Using energy considerations only, determine the
speed of the pot just before it strikes the walk.