Potential and Kinetic Energy
Section 2.2 and 2.3
Review
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What types of energy did we read about last class?
What is gravitational potential energy?
What is kinetic energy?
Objectives
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distinguish between types of potential energy
relate gravitational potential energy to work done
calculate gravitational and elastic potential energy
calculate kinetic energy
Potential Energy
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What is potential energy?
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Energy based on position
Has potential to do work
Gravitational potential energy
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Energy stored based on position above the Earth
Mass versus Weight
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What is the difference between mass and weight?
Mass = scalar quantity
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Weight = vector quantity
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Measured in kilograms (kg)
Measure of gravitational attraction on an object – a force
Measured in Newtons (N)
Fg = mg
g = acceleration due to gravity (9.81 m/s2)
Which one does not change? If I was on the moon (given
moon’s g is 1/6 that of earth), would my mass change or
my weight?
Gravitational Potential Energy and Work
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Gravitational potential energy = work done to lift object
to vertical height
Ep(grav) = W = Fgd
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What are the units of gravitational potential energy?
Ep(grav) = mgh
m= mass (kg)
g= acceleration due to
gravity (9.81 m/s2)
h= height above ground (m)
Example
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A 3.00 kg box is lifted by an upward force 1.50 m above
the surface of Earth to the top of a table. What is the
potential energy stored in the box at its new position?
Answer: 44.1 J
Practice
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A child with a mass of 25.0 kg is at the top of a slide in an
amusement park. If the vertical height of the slide is 4.00
m, calculate the gravitational potential energy of the child
relative to the ground
The Tower of Terror in Disneyland drops its “victims” 13
stories (3 m/story). What would be the potential energy
of a elevator car of 5 people who have a total mass of
116 kg?
Practice
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A 25.0 kg trick-or-treater has run away from a group of
older kids looking to steal her candy. She climbs up a
tree to escape and is measured to have 2.6 kJ of
gravitational potential energy. How high is she above
ground?
Peter Pan is hovering 3.00 m above the ground and has
1.47 x 103 J of gravitational potential energy. Calculate his
mass.
Elastic Potential Energy
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Type of energy stored in a stretched or compressed
spring
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Energy stored in elastic when force used to stretch elastic
(i.e. work done on the elastic or spring)
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Ex. Elastics, trampoline
Stored as potential energy
How do we calculate? Do we use gravity?
What other relationship can we use? (hint we saw it
earlier this class)
Practice
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Robin Hood pulls back his bow to shoot an arrow at King
John’s men. He uses a force of 45 N to pull the string
back 64 cm. Calculate the elastic potential energy in the
bow string.
Chemical Potential Energy
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Store energy in chemicals as potential energy
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Stored in bonds
Energy is released through chemical reactions
Ex. Potential energy of fossil fuels allow us to drive our
cars!
Question
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When is potential energy useful to us?
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Only useful when it is converted to something else!
Ex. Potential energy in a battery useful when converted to
electrical energy
Ex. Gravitational potential energy of a diver useful when start
to dive
Kinetic Energy
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What is kinetic energy?
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Energy of movement
Ek = ½ mv2
Ek = kinetic energy
m = mass (kg)
v = velocity (m/s)
Rearranging the formula…
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In pairs, to be marked, please solve for the following
quantities:
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Mass
velocity
Practice
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A purple-people eater is running after a group of Science
10 students. If its mass is 65.0 kg and it is running at a
velocity of 8.40 m/s, what is its kinetic energy?
Dracula is flying through the night to his castle in
Transylvania at a uniform speed of 5.6 m/s and has a
kinetic energy of 25 J. Calculate his mass in bat form.
Calculate the velocity of Shrek as he runs away from
villagers with pitchforks if his mass is 75.0 kg and his
kinetic energy is 2.78 kJ.
Practice Homework
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P. 178 #2, 6, 8
P. 182 #4, 6, 7