1. Good Morning!
Today we will:
launch pennies into the air
takes some notes
Please do before the tardy bell:
get your lab notebook
get out your notes from this week
get out something to write with
2. Warm-Up
Using the formula for the coefficient of
friction, solve this problem. Put your work in
your notes from this week.
Calculate the μ for wood sliding across carpet
if the weight of the wood is 36N and the
pulling force is 24N.
You have 2 minutes
3. Warm-Up II
Calculate the force of sliding friction for a 500 N
person using a shoe with a μ of 0.4
Hint: use μ = Ff/FN
Calculate the acceleration of the 500 N person
(mass = 50 kg) due to the force of sliding friction.
(Hint: use F = ma)
You have 6 minutes to complete
4. On back of quiz/warm-up
You are a scientist working for ACME Company and
your boss asks you to produce data to answer the
following question:
What is the effect of velocity on the coefficient of
friction between wood and carpet?
Write a hypothesis.
Create a procedure and a data table for the lab
you would conduct to test your hypothesis.
6. Launching Pennies
Hold down one end of the track or wooden ruler
on the table and press down on the other end. Try
to get the penny to travel close to the height of
the ceiling without hitting the ceiling.
What factors (variables) about the track and how
it is positioned determine the height the stone
achieves?
You have 8 minutes
7. Launching Pennies
What is the effect on a penny when additional force
(increased deflection) is applied to the ruler?
your hypothesis
the data you will record
tools you will need to make your measurements
You have 8 minutes
10. Law of Conservation of Energy
When a net force acts on an object, what
happens?
either the speed or position of the object (or
both) change – in other words, the object
accelerates
Think about throwing a ball vertically into
the air. Draw a sketch of what the ball’s
path would look like.
11. Law of Conservation of Energy
The moment the ball leaves your hand, it
has all of the vertical speed it will have –
as the ball rises into the air, what happens
to its vertical speed?
the vertical speed of the ball decreases
12. Law of Conservation of Energy
At some point, the ball will reach its
maximum height. At this point, the ball’s
vertical velocity is zero.
You know what happens next, but do you
know what speed the ball will be when it
reaches your hand again?
when the ball reaches it’s launch height, it will be
traveling at exactly the same speed as it was
when it left your hand.
13. Law of Conservation of Energy
If the speed when you launch the ball is
exactly the same as when the ball returns
back to the same point, then something is
conserved.
What do you think is conserved?
seriously – you better be able to figure out the
answer
14. Law of Conservation of Energy
The Law of Conservation of Energy is
pretty simple:
Energy can be neither created nor
destroyed; it can only be transformed from
one form to another. The total amount of
energy remains constant.
15. Forms of Energy
Energy comes in various forms.
Today, we will be talking about three of
them:
kinetic energy
gravitational potential energy
elastic potential energy
16. Vocabulary Alert!!
kinetic energy is the energy of motion
gravitational potential energy is the
energy of position
elastic potential energy is the energy of a
spring due to compression or stretch
19. Good Afternoon!
Today we will:
finish taking notes on the conservation of energy
diagram, label, and describe energy transformations
use formulas to solve word problems
Please do BEFORE THE TARDY BELL
get out your spiral/notes and look over the definitions
for KE, GPE, and EPE
pick up the “sample problems” worksheet by the door
Pick up a whiteboard and a marker
20. Kinetic + Potential Energy = Total Energy
In any system – whether it’s the ball you
throw vertically in the air or the penny you
launched with the ruler, the total kinetic
energy + the total potential energy = the total
energy in a system
KE + PE = total energy
Our friend Wil E Coyote
21. Kinetic + Potential Energy = Total Energy
Fun with bowling balls
Do you trust physics?
And now for a song you’ll never get out of
your head:
Ole!
22. Concept Check
Think about the lab we did on Monday when we
launched pennies into the air.
elastic potential energy (EPE)
gravitational potential energy (GPE)
kinetic energy (KE)
were all involved in the energy transformations.
23. Concept Check
On the back of the sample problems handout,
sketch the lab we did Monday:
Label the maximum elastic potential energy, the
maximum gravitational potential energy, and the
maximum kinetic energy
Below the sketch, describe the entire path of the
penny in terms of EPE, KE, and GPE and their energy
transformations
You have 12 minutes
24. Concept Check II
True or False: If you know the maximum
kinetic energy in a system, you know the
maximum potential energy as well.
TRUE
25. That was a lot of work! Or was it?
Johnnie pushes against a wall until his
muscles tremble.
Carol Anne picks up her pencil.
Who worked harder?
26. Work
To a scientist, the word work has a very
specific meaning.
Work is defined as a force applied to an
object over a distance.
Work = force x distance
27. Work
So, back to Johnnie and Carol Ann.
Who did more work – Johnnie pushing
against a wall with all of his might or Carol
Ann picking up her pencil?
Let’s “work” a couple of problems
28. UNIT WARNING!
Before we go much further, we need to
emphasize WHAT a Newton is
A Newton is a unit of force that is equal to:
1 kg•m/s2
So BEFORE you start ANY word problem
dealing with work or energy, convert your
units to kilograms, meters, and seconds!
29. Formula for Gravitational Potential
Energy
The formula for gravitational potential
energy is
GPE = mgh
m = mass (kg), g = gravity (m/s2),
h = height (m)
30. Gravitational Potential Energy
GPE = mgh
GPE = Work Done
Work = fd
How are these two quantities related?
mass x gravity = weight (a type of force)
height = a type of distance
What ever GPE an object has, it has it because
your did that much WORK on it.
31. Units
If we solve a gravitational potential energy
problem AND carry our units all the way through
LIKE YOU SHOULD ALWAYS DO,
you end up with a Newton-meter.
GPE = (mass)(gravity)(distance)
(kg)(m/s2)(m)
Nm
Solve sample problem #3 in your handout
32. Formula for Kinetic Energy
The formula to calculate kinetic energy is
KE = 0.5mv2
where m = mass (kg) and v = velocity (m/s)
33. Kinetic Energy
KE = 0.5mv2
looking at the formula, which quantity has
the largest influence on the amount of
kinetic energy – mass or velocity?
Velocity
34. Units
If we solve a kinetic energy problem AND
carry our units all the way through
LIKE YOU SHOULD ALWAYS DO,
you end up with a Newton-meter.
(kg)(m/s)(m/s) = kgm/s2 x m
Nm
Solve sample problem #4 in your handout
35. Elastic Potential Energy
The formula for elastic potential energy is
EPE = 0.5kx2
k = the spring constant (N/m)
x = amount of bending in meters
the spring constant has to be given – it’s
different for different objects
36. Units
If we solve an elastic potential energy problem AND
carry our units all the way through LIKE YOU SHOULD
ALWAYS DO, you end up with something called a
Newton-meter.
EPE = 0.5 (k)(x2)
N/m m2
Nm
You guessed it - time to work some sample problems!
37. Newton-Meters
Energy is measured in Newton-meters
Usually, you’ll see it reported as
something else, though:
A Newton-meter is called a Joule (J)
38. Show How Much You Know
The spring constant for the track we used in this lab is
280 N/m
A penny minted after 1982 has a mass of 2.5 grams
(o.0025 kg)
If you deflect the track 0.03 m (3 cm)
what will be the coin’s maximum velocity
how high in the air will it travel
how long will it stay in the air
no air resistance, penny travels at a 90° angle to the track,
you catch the penny when it returns to the track
