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Part 2, A: THERMODYNAMICS
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WORK
Definition (Young): Energy is the capacity
to do work.
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Part 2, A: THERMODYNAMICS
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WORK
Definition (Young): Energy is the capacity
to do work.
W = Fd
W
F
d
is the work done (Joules)
is the applied force (Newtons)
is the distance moved as a
result of the applied force (meters)
Part 2, A: THERMODYNAMICS
KINETIC ENERGY
Quantity of Motion
KE = ½ m v
2
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Part 2, A: THERMODYNAMICS
KINETIC ENERGY
An object of mass 10 kg has a speed of
10 m/sec. What is the kinetic energy?
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Part 2, A: THERMODYNAMICS
KINETIC ENERGY
An object of mass 10 kg has a speed of
10 m/sec. What is the kinetic energy?
KE = ½ (10 kg) (10 m/sec) (10 m/sec)
500 Joules
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Part 2, A: THERMODYNAMICS
KINETIC ENERGY
An object of mass 10 kg has a speed of
10 m/sec. By how much will the kinetic
energy increase if the speed is doubled?
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Part 2, A: THERMODYNAMICS
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KINETIC ENERGY
An object of mass 10 kg has a speed of
10 m/sec. By how much will the kinetic
energy increase if the speed is doubled?
2
2
2
2
KE = ½ m (2v) = 2 (½ m v ) = 4 (½ m v )
The kinetic energy is 4 times greater.
Energy
Energy is Conserved
KE = W
Energy
Energy is Conserved
KE = W
Work-Energy Theorem
Motion – Newton’s Laws
Special Case: Friction
Example: A force is applied to an object, causing the
object to slide on a table (with friction) at a constant
velocity. The speed is 2 m/sec. If the force is removed,
how far will the block slide before it stops? The
coefficient of kinetic friction is 0.8 and g = 10 m/s2.
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Motion – Newton’s Laws
Special Case: Friction
Example: A force is applied to an object, causing the
object to slide on a table (with friction) at a constant
velocity. The speed is 2 m/sec. If the force is removed,
how far will the block slide before it stops? The
coefficient of kinetic friction is 0.8 and g = 10 m/s2.
NOTE: We already solved this problem in our discussions
about Newton’s second Law.
John E. Erdei, SCI190 Lecture Slides – Newton’s Second Law, Slide 18,
University of Dayton (Unpublished) .
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Motion – Newton’s Laws
Special Case: Friction
Example: A force is applied to an object, causing the
object to slide on a table (with friction) at a constant
velocity. The speed is 2 m/sec. If the force is removed,
how far will the block slide before it stops? The
coefficient of kinetic friction is 0.8 and g = 10 m/s2.
ΔKE = W = F d = µk m g d
½ m v2 = µk m g d
v2 = 2 µk g d
d = v2 / ( 2 µk g) = 0.25 m
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Motion – Newton’s Laws
Special Case: Friction
Example: A force is applied to an object, causing the
object to slide on a table (with friction) at a constant
velocity. The speed is 2 m/sec. If the force is removed,
how far will the block slide before it stops? The
coefficient of kinetic friction is 0.8 and g = 10 m/s2.
Note: This is the same result that we got using the
constant acceleration equations
d = ½ a t2
Δv = a t
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Energy
Potential Energy
Potential energy comes in a variety of
forms.
Energy
Potential Energy
Gravitational Potential Energy
PEGRAV = mgh
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Part 2, A: THERMODYNAMICS
Power
Power is defined to be the rate at which
energy is used.
P = Energy/Time
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Part 2, A: THERMODYNAMICS
Power
P = Energy/Time
Energy
Time
Power
measured in
measured in
measured in
Joules
Seconds
Joules/sec = Watt
Energy
Potential Energy
Football coach Jones becomes angry with a player, and in order
to get the players attention, coach Jones makes the player go
from playing field level up to the top row of seats in the
stadium. Will the player do more work if he walks to the top
row or runs to the top row? Assume the player starts from rest
and stops when he reaches the top.
Energy
Potential Energy
Football coach Jones becomes angry with a player, and in order
to get the players attention, coach Jones makes the player go
from playing field level up to the top row of seats in the
statium. Will the player do more work if he walks to the top
row or runs to the top row? Assume the player starts from rest
and stops when he reaches the top.
There is actually not enough information to determine the work
from first principles. However, since the change in kinetic
energy is 0, the work done by the player must be used to increase
his potential energy (Conservation of Energy). The amount of
potential energy (PE = mgh) is the same, independent of running
or walking, and therefore the amount of work done by the player
is the same if he runs or walks!
Energy
Potential Energy
Football coach Jones becomes angry with a player, and in order
to get the players attention, coach Jones makes the player go
from playing field level up to the top row of seats in the
stadium. Will the player do more work if he walks to the top
row or runs to the top row? Assume the player starts from rest
and stops when he reaches the top.
If this is true, why do they make you run as
punishment, ie, why not punish the player by
making him walk at a leisurely pace to the top of
the stadium????
Energy
Potential Energy
Football coach Jones becomes angry with a player, and in order
to get the players attention, coach Jones makes the player go
from playing field level up to the top row of seats in the
stadium. Will the player do more work if he walks to the top
row or runs to the top row? Assume the player starts from rest
and stops when he reaches the top.
The punishment is related to Power, and not directly to work.
Since
P=W/t
Walking will result in the work to be expended over a longer time
period, therefore requiring less power. Running expends the
work over a shorter time period, therefore requiring more power.