Energy
The ability to make
something happen, to
create a change
I. Types of Mechanical Energy
• A. Potential Energy
• 1. potential energy is stored energy
• 2. Does not involve motion or change
• 3. The object has potential to cause
change
• B. Gravitational Potential Energy (PE)
• 1. determined by its position above the
earth.
• 2. PE = mgh
• m - mass (kilograms)
• g – gravity (9.8 m/s2)
• h – height (meters)
• Both mass and height are
proportional to the amount of
potential energy!
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C. Kinetic Energy
1. the energy of motion
2. Anything moving has kinetic energy
3. KE = 1/2mv2
m – mass (kilograms)
v – velocity (m/s)
• Both mass & velocity are
proportional to the amount of
energy
II. Units of Measurement
• A. Energy is measured in Joules (J)
• B. 1J = kg • m/s2
III. Law of Conservation of
Energy
• Energy cannot be created or
destroyed, but can change forms.
The total of all the energies is
constant.
• Look at the sum of the energies in
the examples.
IV. Mechanical Energy
• 1. The total amount of kinetic and
potential energy in a system.
• 2. ME = KE + PE
• 3. Also measured in Joules
• 4. What happens to the mechanical
energy as KE & PE are converted into
each other?
Example Problems
1. What is the potential energy of a
50N brick on top of a 4m ladder?
Example Problems
2. If a student drops a book having
a gravitational potential energy of
5 joules from a height of 1.1
meters what is the mass of the
book?
Example Problems
3. An SUV with a mass of 2,450
kilograms is traveling at 21
meters/second southeast. What
is the kinetic energy of the SUV?
Example Problems
4. A jet skier with a total mass of
250 kilograms is traveling south
with a kinetic energy of 40,500 J.
What is the velocity of the jet
skier?
Example Problems
5. A cat with a kinetic energy of
160 J is traveling up a tree at a
velocity of 8 meters per second.
What is the cat’s mass?
Practice Problems
1. Calculate the kinetic energy of a 3kg toy
cart that moves at 4m/s.
a. Calculate the kinetic energy of the same
cart at twice the speed.
Practice Problems
2. Calculate the potential energy of a 90N
block of ice sitting on a table 3m above the
ground.
Practice Problems
3. Calculate the potential energy of a
weightlifter holding a 100kg barbell above
his head at a height of 2.2 m.
Practice Problems
4. A 0.5kg sphere at the top of an incline
has a potential energy of 6.0 joules.
How tall is the incline?
a. The sphere rolls halfway down the
incline. What will the object’s new
potential energy be?
Practice Problems
5. A 0.173kg baseball is thrown with a
velocity of 2m/s. What I the kinetic energy
of the baseball?
Practice Problems
6. A 3.1 kg cat sitting on a tree branch has
a potential energy of 153.7 joules. How
far above ground is the tree branch?
Practice Problems
7. A 1000kg roller coaster car sits at the top
of a 30m drop. What is the gravitational
potential energy of the roller coaster car?
Practice Problems
8. When a 5kg mass is lifted from the
ground to a height of 10 meters, the
gravitational potential energy of the mass
is increased by approximately how much?
Practice Problems
9. A 50kg shell is shot from a cannon at
Earth’s surface to a height of 450m. What
is the gravitational potential energy of the
shell at this height?
Practice Problems
a. What is the change in the potential
energy when the shell falls to a height of
225m?
Practice Problems
b. What is the gain in kinetic energy when
the shell falls to the height of 225m?
c. What is the velocity of the shell at this
new height?