Download Conservation of Energy

Do Now:
Determine the change in potential
energy if a 3 kg box is raised
from a height of 1 m to a height
of 3 m.
Do Now:
Complete the Warmup Activity sheet

Do Now (2/1/12):
A force of 56 N is applied to a
box at an angle of 41˚. If the
box moves a distance of 0.23
m, what is the work done on
the box?

CONSERVATION OF
ENERGY
2/2/11
What is
conservation of
mass?
Conservation of Energy:
Energy cannot be created
or destroyed. The total
amount of energy in a
system is conserved over
time.

Conservation of Energy
Etotal  KE  PE
Rollercoaster

http://www.physicsclassroom.com/mmedia/energy/
ce.cfm
Example:
1.
2.
3.
What is the total energy at each given point?
Which type of energy gets larger as the skier falls?
Which type of energy gets smaller as the skier falls?
Do Now (2/2/12):
A 100 kg car on a rollercoaster begins 60 m above
the ground.
1. What is the car’s potential energy at the top?
2. What is the car’s total energy at the top?
3. What is the car’s total energy at the bottom?
4. What type of energy is equivalent to the total
energy at the bottom?
*if you weren’t here yesterday, please
just write the questions!
Review: Conservation of Energy:
Energy cannot be created
or destroyed. The total
amount of energy in a
system is conserved over
time.

Review: Conservation of Energy
Etotal  KE  PE
Strategies for solving Energy problems



Calculate the total energy E as soon as possible
The PE at the reference level is 0 – therefore at the
reference level: Etotal  KE
At the top PE is a maximum – therefore KE is 0 and
Etotal  PE
Practice!

Use the rest of class to practice Conservation of
Energy
LAB!
Read over the lab and
fill out #’s 1 and 5 on
the Results/Calculations
sheet

Do Now:
You lift a 2 kg textbook from the floor to shelf 2.1 m
above the floor.
1.
What is the book’s gravitational potential energy
relative to the floor?
2.
What is its gravitational energy relative to your
head, assuming you’re 1.65 m tall?
Do Now (2/3/12):

In this unit, we learned the food is a
form of stored energy. How many
M&M candies do you think you
would need to replace the energy
you’d burn if you climbed the steps
from the ground floor to the third
floor?
LAB!
Read over the lab and fill out #’s 1 and 5 on the
Results/Calculations sheet
 ONE person from each group will get the hall
pass at a time.
1. Count the stairs
2. Measure the height per stair
3. One person at a time climbs the stairs and times
themselves
4. Everyone else in the classroom should be
working on calculations

LAB! Factor label conversion

To find the energy in EACH candy
 1tbsp   55cal   4190 J 
  
Ecandy  1M & M  
  
  ____ J
 15M & M   1tbsp   1cal 
# Candies 
U g
Ecandy
Strategies for solving Energy problems



Calculate the total energy E as soon as possible
The PE at the reference level is 0 – therefore at the
reference level: Etotal  KE
At the top PE is a maximum – therefore KE is 0 and
Etotal  PE
Practice:

Take 15 min to work on Practice Problems #5-7 on
p. 254 in your classroom textbook. We will go over
them at the end of class.
Example: Conservation of Energy
A 10 kg ball is dropped from a cliff 50 m tall.
1.
What is the ball’s potential energy before being
dropped?
2.
What is the ball’s total energy before being
dropped?
Example: Conservation of Energy
A 10 kg ball is dropped from a cliff 50 m tall.
3. At a height of 30 m, what is the kinetic energy of
the ball?
Example: Conservation of Energy
A 10 kg ball is dropped from a cliff 50 m tall.
4. When it reaches of velocity of 20 m/s, what is the
potential energy?
Do Now (2/6/12):
A 10 kg ball is dropped from a 50 m cliff.
1. What is the potential energy at the top?
2. What is the work done?
3. What is the kinetic energy at the
bottom?
4. What is the velocity of the stone right
before it hits the ground?
Reminders:
TUESDAY 2/7/12 is the LAST DAY to
MAKE UP QUIZZES. Quizzes not made up
by then will be filled in with a score of zero.
 Keep your homework for another night; use
your Conservation of Energy sheet to
make a NOTECARD for tomorrow’s quiz.
 Energy and Power LABS are due tomorrow

Popper Lab
Use TWO HANDS to carry the
balances
 Raise your hand if you have any
QUESTIONS
 DO NOT LOSE YOUR POPPER!!!!!!!!!!!!
This will result in a grade deduction.

Do Now (2/7/12):
1.
2.
3.
4.
In our popper lab yesterday, what form of
energy does the popper have before it
pops?
What was the popper’s initial source of
energy?
Was mechanical energy conserved
throughout the motion of the popper?
What happened to the energy after the
popper stopped moving?
Pass in:
Homework
Labs

Practice:


Use the rest of class to work on “Conservation of
Energy”
On #2 cross off “weighs 20 N”
Do Now:
A 15 kg ball is dropped from rest
with a potential energy of 4600 J.
What is the potential energy when
the velocity is 12 m/s?