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Chapter 5
Pgs. 167-192
1. Identify several forms of energy.
2. Calculate Kinetic energy for an object.
3. Apply the work-energy theorem to solve problems.
4. Distinguish between kinetic and potential energy
5. Classify different types of potential energy.
6. Calculate the potential energy associated with an object’s position.
Energy
Ability to do work
• Kinetic Energy
– Energy of motion = ability to do work
because of motion
• Potential Energy
– Energy of position = ability to do work
because of position within a field force
Kinetic Energy (KE)
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•
•
•
Depends on mass and velocity
Inversely proportional
KE is a scalar quantity
Unit is the joule (J)
A 7.00 kg bowling ball moves at 3.00 m/s.
How much kinetic energy does the ball
have?
How fast must a 2.45 g ping-pong ball
move in order to have the same kinetic
energy as the bowling ball?
The Work-Energy Theorem
• The net work due to all forces equals
the change in the kinetic energy of a
system.
• Wnet = DK
– Wnet: work due to all forces acting on
an object
– DK: change in kinetic energy (Kf – Ki)
Work – Kinetic Energy
Theorem
∆KE = KEf – KEi
A 65 kg crate is pushed along a horizontal surface
by two students. After the crate is pushed a
distance of 6.0 m, starting from rest, its speed is
4.5 m/s. Find the magnitude of the net force on the
crate.
Potential Energy (PE)
• Gravitational Potential Energy
– Energy due to the object’s position relative
to a zero level
Gravitational Potential Energy
• Depends on mass, height, and
acceleration due to gravity
• Unit is the joule (J)
A spoon is raised 21.0 cm above a table. If the
spoon and its contents have a mass of 30.0 g,
what is the gravitational potential energy
associated with the spoon at that height
relative to the surface of the table?
Potential Energy (PE)
• Elastic Potential Energy
– Energy due to the object’s position when it is
stretched or compressed
Elastic Potential Energy (PEe)
Elastic Potential Energy (PEe)
•k is the spring constant
•units of newtons divided by meters
(N/m)
•x is the distance compressed or
stretched
•usually the difference between
unstretched and stretched lengths
Springs
• When a spring is stretched or compressed from its equilibrium
position, it does negative work, since the spring pulls opposite
the direction of motion.
• Ws = - ½ k x2
– Ws: work done by spring (J)
– k: force constant of spring (N/m)
– x: displacement from equilibrium (m)
• The force doing the stretching does positive work equal to the
magnitude of the work done by the spring.
• Wapp = - Ws = ½ k x2
A stuntman is attached to a bungee cord with an
unstretched length of 15.0 m. He jumps off a bridge
spanning a river from a height of 50.0 m. When he
finally stops, the cord has a stretched length of 44.0
m. Assuming the spring constant of the bungee cord
is 71.8 N/m, what is the elastic potential energy?
Other Forms of Energy
Thermal energy – heat
c = Q/(m•Δt)
Electrical energy – electricity
Ue = K[(q1•q2)/r]
Chemical energy – stored in bonds, vander waals forces
Body uses all types of energy converting one into another
food energy stored in bonds Calories
1 Calorie = 4180 Joules
Can convert mass to energy by
fusion – combining mass
fission – breaking down mass