1. Work and Energy Conservation of Energy
Chapter 5: Section 3

2. Learning Targets
Identify situations in which conservation of mechanical energy is valid
Recognize the forms that conserved energy can take
Solve problems using conservation of mechanical energy
P4.3A, P4.3B, P4.3e

3. Kinetic and Potential Energy
The motion of many objects often contain a combination of kinetic and potential energy
At the highest point of a swinging pendulum there is only gravitational potential energy
At other points, the pendulum is in motion, so it has kinetic energy
Elastic potential energy is also present in the many springs that are part of the inner working of the clock

4. Σ represents the sum gravitational and elastic PE
Mechanical Energy
Mechanical energy is the sum of kinetic energy and all forms of potential energy associated with an object or group of objects
Mechanical energy is the ability to do work
ME = KE + ΣPE
Σ represents the sum gravitational and elastic PE
All energy, such as nuclear, chemical, internal, and electrical, are classified as non- mechanical energy

5. Internal vs. External Forces
All forces in physics can be classified as either internal or external
External forces include the applied force, normal force, tension force, friction force, and air resistance.
Internal forces include gravity, magnetic force, electrical force, and spring force.

6. Conserved Quantities
In the absence of external forces, mechanical energy is always conserved
When something is conserved, it remains constant
This does not mean that the quantity cannot change form during that time,
A dollar can be 10 dimes, 100 pennies, or 4 nickels
A glass can break, but all of the pieces add up to the same mass of the glass as a whole

7. Conservation of Mechanical Energy
In the absence of friction, including air resistance, the total mechanical energy remains constant
The principle is called conservation of mechanical energy
½ mvi2 + mghi = ½ mvf2 + mghf
In the case that there is elastic potential energy the term ½ kx2 would be added in

8. As an egg falls, the potential energy is converted into kinetic energy
If the egg were thrown up in the air, the kinetic energy would be converted into gravitational potential energy
In situations in which frictional forces are present, mechanical energy conservation no longer holds because kinetic energy is not simply converted to potential energy

10. Mechanical Energy and Friction
In the presence of friction, non-mechanical energy is no longer negligible and mechanical energy is no longer conserved
This does not mean that energy is not conserved because the total energy is always conserved
When mechanical energy is converted into non-mechanical forms of energy, this energy becomes much more difficult to account for, and the mechanical energy is considered to be lost