Download Holt Physics—Chapter 5: Work and Energy

Holt Physics—Chapter 5: Work and Energy
I.
Price
Section 5.1—Work
A. Definition of work
1. Work does not mean the same thing in Physics as it
does in the everyday sense of the word.
2. Work is defined as a force causing a displacement.
Work = force x displacement
W = Fd
3. Work is NOT done on an object unless the displacement
is greater than zero
4. The only forces that are considered to do work are
those that are parallel to the displacement.
5. For this reason we use our trigonometric functions to
calculate forces applied at an angle.
Insert Fig 5-2
6. Note that Θ is the angle between the applied force and
the displacement.
7. Work is described in Newtons x meters (force x
displacement). The unit of work is the Joule (J)
8.
1 Newton meter = 1 Joule
9. Work is a vector with both direction AND magnitude.
This means WORK CAN BE NEGATIVE!
10. Negative work is most commonly used to slow an object
down or decrease its velocity.
1
II.
Section 5-2: Energy
A. Kinetic Energy
1. Kinetic energy is associated with an object in motion.
2. Kinetic energy depends on speed and mass
Kinetic Energy = ½mv2
3. Kinetic energy is a scalar and will use Joules as its
standard unit.
4. It takes some work to change an object’s kinetic energy
either by changing the velocity or changing the mass.
5. This fact leads us to the Work-Kinetic Energy Theorem
WORK-KINETIC ENERGY THEOREM
Wnet = ∆KE
Net work = change in kinetic energy
6. Notice that Wnet = ∆KE
and
Wnet = Fnetd(cosΘ)  (from section 1)
therefore,
∆KE = Fnetd(cosΘ)
B. Potential Energy
1. Kinetic energy is associated with
motion…potential energy is stored energy with
the potential to do work given the right
conditions.
a. Examples: a ball at the top of a ramp,
an arrow in a bent bow, a rocket on a
launch pad
2
2. Gravitational Potential Energy is the result of
an object’s position relative to a gravitational
field. (also measured in Joules)
Gravitational Potential Energy
PEg = mgh
Gravitational Potential Energy = mass x gravity x height
3. “Gravitational potential energy is the result of
an object’s position…relative to some zero
level.” (p.178) This means that we compare an
object’s potential energy relative to a position
where its potential energy would be zero.
4. Elastic Potential Energy can be stored in
springs and elastic.
Elastic Potential Energy
PEelastic = ½kx2
Elastic Potential Energy = ½spring constant x distance
stretched or compressed
a. k = the spring constant and is different
for every spring (weak springs have a
small k, strong springs have a large k)
and are measured in N/m.
b. x = the distance the object is stretched
or compressed in meters
(insert fig. 5-8)
3
III. Section 5-3: Conservation of Energy
A. The First Law of Thermodynamics is that “In any
process, the total energy of the universe remains
constant.”—Wikipedia
This means that energy
can never be created or destroyed only converted into
mass and back again. (E = mc2)
B. Mechanical Energy (kinetic and potential energy) is
often conserved if we neglect friction.
C. Energy can change form (from potential to kinetic, to
gravitational to elastic, but will always total up to be
the same
D. Sample Problem 5E on p.184 provides a good example
E. Friction converts mechanical energy (movement) into
non-mechanical energy (heat)…ergo mechanical energy
is NOT conserved in the presence of friction, but
total energy is.
IV. Section 5-4: Power
A. The rate at which work is done is called power
Power
P = W/Δt
Power = work / time
B. Since work (W) is defined as Force x distance we can
rewrite P = Fv (power = force x velocity)
C. The standard unit for power is the watt
D. Conversions:
1 watt = 1 joule/second
1 horsepower = 746 watts
4