Do Now
Materials
Notebook
Homework
Pen/Pencil
1) Take Out Your Homework, Put on Corner of Desks
2) In Notebooks: What is Power?
-Write a sentence in response.
3) In Notebooks: When Mr. Van Houten bench
presses large amounts of weight he often will do
500 J of work in just 4 seconds. How much power
do I develop?
Objectives
• SWBAT solve simple power problems.
• SWBAT solve work-energy theorem problems.
Agenda
• Introduction to New Material: Work – Energy
Theorem
• Practice
• More Practice
• Exit Ticket
SWBAT solve simple power problems.
SWBAT solve work-energy theorem problems.
Work – Energy Theorem
• So far our exploration of energy has been so
exciting.
– A box sits on a cliff, how much energy does it have?
– A spring shoots a dart straight up into the air, what’s
the change in energy?
– Your moving in a car how much kinetic energy do you
have?
SWBAT solve simple power problems.
SWBAT solve work-energy theorem problems.
Work – Energy Theorem
• We’ve been talking about the energy of
objects that are in closed systems.
• For example, one system might be a box in a
warehouse.
• We haven’t considered the external world that
may affect the box.
• The box has no energy if it sits there, but if I
pick it up and put it on a shelf…
SWBAT solve simple power problems.
SWBAT solve work-energy theorem problems.
Work – Energy Theorem
• Through the process of doing work, energy can
move between the external world and the
system.
• The direction of this energy transfer can go
both ways.
• External world
system
– I lift a weight off the ground and hold it above my
head.
• Or, system
external world
– A moving golf club hits a still golf ball.
SWBAT solve simple power problems.
SWBAT solve work-energy theorem problems.
Work – Energy Theorem
Weight is
the
system
Work – Energy Theorem
• External world
system
– I lift a weight off the ground and hold it above my
head.
– W is positive
Ei  W  E f
– Mr. Van Houten picks a weight up off the ground by
doing 50 J of work on the weight. How much
energy does the weight now have?
Ei  W  E f
0  50  50 J
Work – Energy Theorem
• Or, system
external world
– A moving golf club hits a still golf ball.
– W is negative
Ei  W  E f
– A 2 kg golf club is being swung at 4 m/s, when it
collides with a golf ball. During the collision, the
club does 5 kg of work on the ball. What is the final
energy of the golf club equal to?
Ei  W  E f
KE  5   E f
Work – Energy Theorem
• The work-energy theorem states that when
work is done on an object, the result is a
change in kinetic energy.
• The work-energy theorem can be represented
by:
W  E
W  E f  Ei
• Think back to Mr. Van Houten lifting the
weight.
SWBAT solve simple power problems.
SWBAT solve work-energy theorem problems.
Work – Energy Theorem
Weight is
the
system
Work – Energy Theorem
• The relationship between work done and the
change in energy that results was established
by James Prescott Joule.
– To honor him, a unit of energy is called a joule (J).
SWBAT solve simple power problems.
SWBAT solve work-energy theorem problems.
Example Problem #1
• A 105-g hockey puck is sliding across the ice.
A player exerts a constant 4.50-N force over a
distance of 0.150 m. How much work does
the player do on the puck? What is the
change in puck’s energy?
Known
• m=105 g
• F=4.50 N
• d=0.15 m
Unknown
• W= ?
• ∆E= ?
d
F
Example Problem #1
• A 105-g hockey puck is sliding across the ice.
A player exerts a constant 4.50-N force over a
distance of 0.150 m. How much work does
the player do on the puck? What is the
change in puck’s energy?
Known
Unknown
W  Fd cos(  )
W  ( 4 . 50 )( 0 . 150 ) cos( 0 )
• m=105 g
• F=4.50 N
• d=0.15 m
• W= ?
• ∆E= ?
W  0 . 675 J
Example Problem #1
• A 105-g hockey puck is sliding across the ice.
A player exerts a constant 4.50-N force over a
distance of 0.150 m. How much work does
the player do on the puck? What is the
change in puck’s energy?
W  E
Known
Unknown
 KE  0 . 675 J
• m=105 g
• W= 0.675J
Does the sign make
• F=4.50 N
• ∆E= ?
sense?
• d=0.15 m
Example Problem #2
• Juan lifts a potted plant into the air. If he gives
the plant 90 J of potential energy in 0.65
seconds, how much power did Juan develop?
Known
•
•
•
•
t=0.65 s
∆E=90 J
W=∆E
W=90 J
Unknown
• P= ?
Example Problem #2
• Juan lifts a potted plant into the air. If he gives
the plant 90 J of potential energy in 0.65
seconds, how much power did Juan develop?
Known
•
•
•
•
t=0.65 s
∆E=90 J
W=∆E
W=90 J
Unknown
• P= ?
P 
W
t
P 
90
0 . 65
P  138 . 46 W
Example Problem #3
• A skater with a mass of 52 kg moving at 2.5
m/s glides to a stop over a distance of 24.0 m.
How much work did the friction of the ice do
to bring the skater to a stop?
Known
• m=52 kg
• Vi=2.5 m/s
• Vf=0 m/s
• d=24 m
• W=∆E
Unknown
• W=?
Example Problem #3
• A skater with a mass of 52 kg moving at 2.5
m/s glides to a stop over a distance of 24.0 m.
How much work did the friction of the ice do
to bring the skater to a stop? W   E
W  E f  Ei
Known
• m=52 kg
• Vi=2.5 m/s
• Vf=0 m/s
• d=24 m
• W=∆E
Unknown
• W=?
W  0  KE
W 0
1
mv
2
2
W 
1
2
( 52 )( 2 . 5 )
2
W   162 . 5 J
Rest of Class
• Power Practice Worksheet
– Work quietly with 1 other person
SWBAT solve simple power problems.
SWBAT solve work-energy theorem problems.
Homework: Due Thursday
• Finish Power Practice
• Physics Update #3
– Based off notes from today.
SWBAT solve simple power problems.
SWBAT solve work-energy theorem problems.