conservation of energy for web
TRANSCRIPT
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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
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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
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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
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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.
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What Do You See?
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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
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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
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Launching Pennies
Perform your experiment
You have 10 minutes
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Law of Conservation of Energy
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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.
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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
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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.
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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
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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.
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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
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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
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End Day 1 Notes
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Day 2 Notes
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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
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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
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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!
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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.
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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
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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
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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?
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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
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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
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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!
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Formula for Gravitational Potential Energy
The formula for gravitational potential energy is
GPE = mgh
m = mass (kg), g = gravity (m/s2),
h = height (m)
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Gravitational Potential Energy
GPE = mgh
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.
GPE = Work Done
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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
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Formula for Kinetic Energy
The formula to calculate kinetic energy is
KE = 0.5mv2
where m = mass (kg) and v = velocity (m/s)
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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
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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
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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
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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!
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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)
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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