Download Ch 05.H Notes 2016

Holt Chapter 5 Class Notes:
Work is done on an object when a FORCE is applied
which moves that object through a DISTANCE.
Work, Energy and Power
Is work being done in this picture?
The amount of work done equals the distance moved times
the component of the force that acts in the direction of
motion.
in other words... or symbols...
W= FdCosθ
If the force and displacement are in the same direction this
simplifies to W = Fd (why?)
ce
for
d
lie
app
horizontal component
W= FdCosθ
Only the component of the force that acts in the
direction of motion accomplishes work.
How much work is done?
W= FdCosθ
Only the component of the
force that acts in the direction of
motion accomplishes work.
Work can also be found by calculating the area under a Force vs.
displacement graph.
How much work is done?
W= FdCosθ
Only the component of the
force that acts in the direction of
motion accomplishes work.
Work can also be found by calculating the area under a
Force vs. displacement graph.
Units
kg•m2
s2
W= FdCosθ
F = ma = (kg)(
m
) and
s2
The units of work are
N•m
Joule (J)
d =m
kg•m2
s2
Stop Day
One
The sign of work is important!
The sign of work is important!
The work done on an object is positive when the component
of force is in the same direction as the displacement.
The work done on an object is negative when the component
of force is in the opposite direction as the displacement.
(θ < 90°)
(θ < 90°)
See Figure 3 in Holt page 162.
See Figure 3 in Holt page 162.
Energy can be defined as “the capacity to do work”.
Work is a measure of the Energy converted from
one form to another.
Energy is a conserved quantity.
Kinetic energy is the energy of motion.
Energy can be converted from one form to another
but the total amount of energy will stay constant.
Kinetic energy is defined as:
1
KE = mv
2
2
Work-Kinetic Energy Theorem
Work-Kinetic Energy Theorem
Sometimes called the Work-Energy principle:
“The Net Work done on an object is equal to the
object’s change in kinetic energy.”
First, let’s define net work.
W net = Fnet d
Work-Kinetic Energy Theorem
The Work-energy principle
The Work-energy principle:
When work is done on an object, the
object’s kinetic energy will change.
W = ΔKE
net
W net = KE f − KE i
1
1
W = mv − mv
2
2
2
net
f
i
If W is positive, KE will increase.
2
If W is negative, KE will decrease.
Gravitational Potential Energy:
Potential Energy (PE):
The energy associated with an object due to the
position of the object.
You can think of potential energy as energy that is
stored.
When you lift an object against gravity, you increase
that object’s gravitational potential energy.
PE
gravitational
= mgh
The Potential Energy associated with an object due
to the position of the object relative to the Earth
or some other gravitational source.
When you lift an object against gravity, you increase
that object’s gravitational potential energy.
h1
h3
h2
Where h is the height above some arbitrary reference
point ... usually the ground.
PE
gravitational
= mgh
1
KE = mv
2
2
Where h is the height above some arbitrary
reference point ... usually the ground.
Elastic Potential Energy:
The Potential Energy stored in a stretched or
compressed elastic object.
Elastic Potential Energy:
Energy stored
in spring
PE
Spring constant
(N/m)
elastic
1
= kx
2
2
Distance spring is
stretched or
compressed
Elastic Potential Energy:
Energy
Mechanical
Kinetic
Nonmechanical
Potential
Gravitational
Potential
The Law of Conservation of Energy
Energy can be transferred from one form to
another, but the total energy in a closed system
remains constant.
For objects affected only by gravity and in the
absence of friction, we have a simplified version of
conservation of energy:
Power
PEi + KEi = PEf + KEf
1
1
mgh + mv = mgh + mv
2
2
2
i
i
f
2
f
Power
Power is the rate at which work is done.
Power is not a measure of how much work is done.
Power is a measure of how quickly work is being done.
Power
Power is the rate at which work is done.
W FdCosθ
P= =
Δt
Δt
Another way to calculate Power:
Fd
d
P=
= F = Fv
Δt
Δt
Power is measured in Joules per second.
One Joule per second is also called 1 Watt.
J
1 = 1W
s
P=
W FdCosθ
=
Δt
Δt
Fd
d
P=
= F = Fv
Δt
Δt
A commonly needed conversion:
1 Horsepower (HP) = 746 Watts
Horsepower is not an SI unit but it is
commonly used in the US
The End