Download Physics--Chapter 5: Work and Energy

Physics--Chapter 5: Work and Energy
Lecture Notes
I.
II.
Work
A. Definition of Work
1. Work is defined as the product of the magnitudes of the component of a
force along the direction of displacement and the displacement
2. Work = force x distance (W = F  d, or Wnet = Fnet  d)
3. Work is not done on an object unless the object is moved due to the action
of the force
4. Work is only done when components of a force are parallel to a
displacement
B. The unit of work is the joule or Newton-meter
Unit Analysis for Work:
C. Work is a scalar quantity and can be positive or negative
1. if force applied helps motion, work is positive
2. if force applied opposes motion, work is negative
Energy
A. Kinetic Energy
1. Defined as the energy of an object due to its motion (an object must be
moving to have kinetic energy)
2. Kinetic Energy (KE) = ½ mv2 (where m = mass and v = velocity)
3. Units of KE are joules
Unit Analysis for Kinetic Energy:
B. Potential Energy
1. Defined as the energy associated with an object due to its position;
sometimes simply thought of as stored energy (something has potential
energy if it has the ability to do work)
2. Types of Potential Energy
a. gravitational potential energy
1) defined as the potential energy associated with an object due to its
position relative to the earth or some other gravitational source
2) gravitational potential energy (PE g) = mgh (where m =mass, g =
free-fall acceleration, and h = height)
3) when calculating gravitational potential energy, a zero level must
be defined (may or may not be the ground)
4) units of PEg are joules
Unit Analysis for Gravitational Potential Energy:
b. elastic potential energy
Physics--Chapter 5: Work and Energy
Lecture Notes
1) defined as the potential energy in a stretched or compressed elastic
object
2) elastic potential energy (PEelastic) = ½kd2 (where k = spring or force
constant and d = distance compressed or stretched)
3) spring constant is defined as the parameter that expresses how
resistant a spring is to being compressed or stretched, measured in
N/m
4) units for PEelastic are joules
Unit Analysis for Elastic Potential Energy:
III.
IV.
c. also, if situation has both, PEtot = PEg + PEelastic
d. other types of potential energy??
C. Mechanical Energy
1. defined as the sum of kinetic energy and all forms of potential energy
2. ME (mechanical energy) = KE + ΣPE
3. Nonmechanical energy is all energy that is not mechanical (can you think
of an example??)
Conservation of Energy
A. Just as mass is conserved (law of conservation of mass), mechanical energy is
also conserved in the absence of friction
B. Law of Conservation of Energy--energy cannot be created or destroyed
C. Mechanical energy can change forms, but is conserved
D. MEi (initial mechanical energy)= ME f (final mechanical energy)
E. Mechanical energy is not conserved in the presence of friction because
friction sources convert mechanical energy into nonmechanical forms (yet,
total energy is conserved!)
Work, Energy, and Power
A. The Work-Kinetic Energy Theorem
1. States that the net work done on an object is equal to the change in the
kinetic energy of the object
2. Equationally: Wnet = KE, net work = change in kinetic energy
3. Alternatively, when work is done by friction, Wfriction = ME
a. if problem does not involve friction, the W friction = 0
b. then, Wfriction = ME becomes Wfriction = 0, so ME = 0, so MEi = MEf
B. Power
1. Defined as the rate at which work is done, or the rate at which energy is
transferred
2. Equationally: Power = work ÷ time, P = W/t
3. Since W = F × d, P = W/t = Fd/t = Fv, so Power is also equal to force ×
velocity, P = F × v
4. Power is measured in watts (W), which are equal to J/sec; also 1 hp
(horsepower) = 746 W
Unit Analysis for Power: