Energy

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# Energy - PowerPoint PPT Presentation

Energy. Bellringer. Write your name on a piece of loose leaf paper. Take out your lab from last year Pendulum Pun. Objectives. Finish the lab Analyze Your Grade Race Track Demo. Grades. If the quarter ended right now, would you be happy with your grade?

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### Energy

Bellringer
• Write your name on a piece of loose leaf paper.
• Take out your lab from last year
• Pendulum Pun
Objectives
• Finish the lab
• Race Track Demo
• If the quarter ended right now, would you be happy with your grade?
• If you’re not, what will you do to change it?
Race Track Demo
• To the back
Bellringer
• What is the only variable that changes the period of a pendulum?
Objectives
• Learn about the different forms of energy and how to use them to solve problems.
The Many Forms of Energy
• What are some forms of energy that you have heard of before?
• We’re going to start be focusing on two types of energy.
• The unit of energy is J for Joule
Kinetic Energy
• What does kinetic mean? (Think about kine-matics, and kinetic friction)
• Kinetic Energy is the energy of a moving object.
• Every moving object has kinetic energy.
Kinetic Energy
• If a puck with a mass of 2kg is sliding on frictionless ice at a constant velocity of 5m/s. What is its kinetic energy?
• 25J
• If an object is accelerating, how is its kinetic energy changing?
• Its kinetic energy is increasing
Potential Energy
• Energy that is stored due to interactions between objects in a system is called potential energy.
• Gravitational Potential Energy is the energy stored in an object due to gravity
• GPE=mgh
Gravitational Potential Energy
• How do you store GPE in an object?
• To store GPE you simply increase the distance of the object from the reference level.
• Reference level: A position that you chose to define the GPE as zero.
Reference Level
• Where should the reference level be for the roller coaster below?
Gravitational Potential Energy
• Which has more GPE: a 9lb bowling ball that is 2m above the ground or a 9lb bowling ball that is 4m above the ground?
• The bowling ball that is 4m above the ground will have twice as much GPE as the bowling ball 2m above the ground.
Transformation of Energy
• Money analogy
• Pendulum demo
Conservation of Energy
• The law of conservation of energy states that in a closed, isolated system, energy can neither be created nor destroyed; rather, energy is conserved.
• Under these conditions, energy can change form but the system’s total energy in all of its forms remains constant.
Conservation of Energy
• Total energy
• KE = Kinetic energy
• PE = Potential energy
• Q = Internal energy
• Internal energy can be a variety of things.
• Explains the loss of energy to friction
Conservation of Energy
• After an arrow is fired from a bow it has 300J of kinetic energy. If energy was conserved, what was the potential energy stored in the bow before the arrow was shot?
Conservation of Energy
• Is energy conserved in a pendulum?
• If a 6.8kg bowling ball is 1.0m above its lowest point in its swing, what is its maximum velocity?
• Draw a diagram.
Conservation of Energy
• Race Track Demo
• Where should the two marbles land, and why?
Conservation of Energy
• Pin in Pendulum demo
• Is the period of the pendulum the same on both sides?
• What evidence shows us that energy is still conserved even though the period is changing?
Conservation of Energy
• Magic Cleaning Can
• Use the law of conservation of energy to explain why the can comes back to me.
Conservation of Energy
• Dropper Popper
• Use the law of conservation of energy to explain how you can predict the how high the popper will jump.
Conservation of Energy
• Balloons
• Use the law of conservation of energy to explain why the balloon flies away when it is released.
• Use the law of conservation of momentum to explain why the balloon flies away when it is released.
Classwork
• Read Chapter 11 for clarification.
• One page 312-313 answer the following questions:
• 33,37, 40,49,51, 58, 59, & 64.
Bellringer
• What is the velocity of a pendulum at the bottom of its path if it is released from 2 meters above its lowest point?
• v=6.3m/s
Objective
• Lab # 14
• Introduction to the next part of energy
Homework
• Energy HW Packet!
• Three short homework assignments.
• All due Monday 1/13
• Collected
• Counts as three homework assignments
Bellringer
• Explain how energy is conserved in a simple pendulum.
• Skate park energy conservation demo
Objectives
• Finish Lab 14 and go over the answers to the post lab questions
• Investigate springs, and compare them to what we have already learned about pendulums
Homework
Midterm
• Midterm will be either the last week of January or the first week of February
• It will cover all of mechanics
• 1-D and 2-DKinematics, Dynamics & Statics, Work & Energy, Momentum and Impulse, Uniform Circular Motion, Newton’s Universal Law of Gravitation, and Measurement & Math
Midterm
• Two mods long
• Multiple Choice and Long Answer
• All Regents type questions
• “College” Midterm
• You will use everything I’ve given you to study.
• We won’t review in class for it.
• See me if you have questions or need help.
Lab 14
• Finish collecting your data then disassemble everything and leave it on your table.
• We will answer the post-lab questions together.
Bellringer
• What variable of a pendulum should you change to maximize its velocity at the bottom of its swing?
Objectives
• Discover another useful way of continually transforming energy between kinetic and potential energy.
Bellringer
• Explain how energy is conserved in a simple pendulum.
• Skate park energy conservation demo
Objective
• Investigate springs, and compare them to what we have already learned about pendulums
Homework
Midterm
• Midterm will be either the last week of January or the first week of February
• It will cover all of mechanics
• 1-D and 2-DKinematics, Dynamics & Statics, Work & Energy, Momentum and Impulse, Uniform Circular Motion, Newton’s Universal Law of Gravitation, and Measurement & Math
Midterm
• Two mods long
• Multiple Choice and Long Answer
• All Regents type questions
• “College” Midterm
• You will use everything I’ve given you to study.
• We won’t review in class for it.
• See me if you have questions or need help.
Pendulums
• Are pendulums the only simple system that easily show how energy can be transformed repeatedly between potential and kinetic while being conserved?
• NOT A CHANCE!!!!
SPRINGS!!!!
• Like pendulums, springs can also display periodic motion!
• Periodic Motion: Motion that repeats in a regular cycle.
• Springs can be used to show Simple Harmonic Motion!
• SHM: Any system in which the force acting to restore an object to its equilibrium position is directly proportional to the displacement of the object.
Hooke’s Law
• Robert Hooke was a British physicist who discovered the simple harmonic motion of springs.
• He published his discovery in 1660 in a Latin anagram.
• He later published the solution in 1678
Hooke’s Law
• Hooke’s Law states that a spring exerts a force directly proportional to the distance it is stretched.
Hooke’s Law’s Units
• The negative sign in Hooke’s Law indicates that the force is in the direction opposite the stretch or compression direction (always towards equilibrium).
• The larger the k value, the stiffer the spring.
Hooke’s Law Example
• What is the force of a spring that has a spring constant of 67N/m and is stretched 0.25m
• 16.75N towards equilibrium
• What is the spring constant of a spring that is compressed 0.33m away from its equilibrium position with a force of 12N?
• 36.4N/m
Hooke’s Law is FLAWED!!
• Hooke’s law does not work for every spring (i.e. rubber bands).
• Springs that obey Hooke’s law are called elastic springs.
• The Elastic Limit of a spring
• Metal springs usually can only be stretched or compressed so much before they deform.
Spring Potential Energy
• Springs can also store potential energy
• What happens to the potential energy of a spring as you compress or stretch it more and more?
• It increases, until it becomes too much and breaks…
Spring Kinetic Energy
• Pssh didn’t you read the reference table? Springs don’t have a special kinetic energy.
• The springs we use are “massless” so the mass and velocity here come from the object in contact with the spring.
Online Demo
• No friction
• 3rd spring (3/4 hard)
• 1/16 time
• G=0
• Draw bar graphs of the Total Energy, Potential Energy, and Kinetic Energy of the red block – spring system for all four stage shown.
Questions
• Where is the block’s velocity zero?
• Where is the blocks velocity maximized?
• Where is the block’s acceleration zero?
• Where is the block’s acceleration maximized?
Slow-Mo Springs
• What shape is this?
• Sine Wave!!!
Practice Problems
• One Page 385 of the textbook answer questions 1, 2, 3, and 4.
• 200 N/m
• 1.96J
• 0.32m
• 0.61m
Homework
• 38-40, 44-48, 50
Bellringer
• Name two examples of periodic motion?
Objectives
• Review what we learned yesterday about springs
• Begin an investigation into springs
Go Over Homework
• 38: Simple harmonic motion is periodic motion that results when the restoring force on an object is directly proportional to its displacement. A block bouncing on the end of a spring is one example.
• 39: The spring stretches a distance that is directly proportional to the force applied to it.
• 40: The spring constant is the slope of the graph of F versus x.
Go Over Homework
• 44: 27N/m
• 45: 0.12m
• 46: 0.35J
• 47: 0.29m
• 48: a. 20N/m
• b. 2.5J
• 50: A=B<C=D
Energy Homeworks
• If you missed the three energy homework assignments your grade dropped between 5 and 10 points…
Lab 15
• Take a moment to read the beginning of the lab, then meet me in the back.
Bellringer
• Three Spring Demo
Objectives
• Finish Hooke’s Law Lab
• Go over Energy Conservation HWs
• Learn, and apply what us physicists like to call “The Work, Energy Theorem”
Note
• Someone pointed out to me yesterday that on the reference table Hooke’s Law does not have a negative sign…
• This is ok. The negative sign is just used to show it’s a force against the force you apply.
• You don’t have to use the negative sign.
Hooke’s Law Lab
• Grab your lab partners and finish up that lab.
• All of it (graph, and questions) is due at the end of the mod.
• Let me know if you have any questions.
Homework
• After looking over you 3 Energy HWs, what questions do you have?
Total Energy
• We already know that the total energy of a system (pendulum, spring, etc.) remains the same.
• Is it possible increase a systems total energy?
• Yes you just take energy from another system and transfer into the system you are studying.
Changing Total Energy
• When a closed system’s total energy is changed it is said that “Work was done” on that system.
• Work done on a system is equal to the change in the system’s energy
• (The Work Energy Theorem)
• Work is in Joules (Same as energy)
Work
• For example: Bobsledders must do work on their sled to get it moving at the beginning of a race.
• Every time you lift something up (going against gravity) you are doing work against gravity.
Work
• If the KE of a system is changed, work is done to change the object’s velocity.
• If the PE of a system is changed, work is done to change the object’s height.
• If the Q of a system is changed, work is done by friction.
Work Up Ramps
• Does it take less work to increase something’s height if you use a ramp instead of just lifting it straight up?
Inclined Plane
• What is the work done on a 10kg ball that is rolled up a frictionless ramp at constant velocity
• Same work
• As lifting it
• Straight up!
Example
• How much work must you do on a 5.0kg box to increase its velocity from 0 m/s to 16 m/s on a frictionless horizontal floor?
Example
• How much work is done in order to lift a 3.0kg paint bucket 2.0m above the ground at constant velocity?
Try It!
• Your 75kg friend wants to be pushed on a swing. Ignoring friction how much work must you do in order to raise your friend 1.5m at constant velocity?
Calories are a FRAUD!!!!
• Calories are a unit of energy that you commonly see associated with food.
• One calorie is 4.184 Joules
• How many calories did you “burn” pushing your friend in the previous problem?
Classwork
• The 3rd floor of a house is 8m above street level. How much work must a pulley system do to lift a 150-kg oven at a constant speed to the 3rd floor?
• 11,772 Joules
• Haloke does 176 J of work lifting himself 0.300m at a constant speed. What is Haloke’s mass?
• 59.8 kg
Bellringer
• The 3rd floor of a house is 8m above street level. How much work must a pulley system do to lift a 150-kg oven at a constant speed to the 3rd floor?
• 11,772 Joules
• Haloke does 176 J of work lifting himself 0.300m at a constant speed. What is Haloke’s mass?
• 59.8 kg
Objectives
• Understand why is it easier to use and ramp instead of just lifting something vertically.
• Work on work practice problems.
Note
• We use the term “calorie” completely wrong.
• When ever you think of a calorie you are actually thinking of a kilocalorie or 1,000 calories…
• The man is bringing us down!
Swing Question Take Two
• Your 75kg friend wants to be pushed on a swing. Ignoring friction how much work must you do in order to raise your friend 1.5m at constant velocity?
• 1,100 Joules
• How many kilocalories did you burn?
Work Up Ramps
• If you do the same amount of work, why is it easier to use a ramp instead of lifting something vertically?
Work
• To understand this phenomena we need to know more about work.
• Work can be calculated two ways
• F=Force (Newtons)
• d=displacement (meters)
Work done against gravity
• This isn’t much different than what you have already done with the change in PE being the change in total energy.
Work done horizontally
• We know an unbalanced force will produce an acceleration.
• So if an object is accelerating its velocity is changing.
• If the velocity is changing then its kinetic energy is changing.
So…
• Hopefully it is more understandable how force and work are related, but anyway, we can write:
• This is the form that is written on your reference table.
Example
• What is the work done by a 50N force over 10m?
• What is the change in energy caused by this force?
• 500J
You try!
• A hockey player uses a stick to exert a constant 4.50N force forward to a 105g puck sliding on ice over a displacement of 0.150m forward. How much work does the stick do on the puck? Assume friction is negligible.
• =0.675J
• Do you need the mass for this problem?
• Nope
Back to the inclined plane
• What is the work to move a box up a frictionless slope that is “a” long?
Back to the inclined plane
• What is the work to move a box vertically up the same inclined plane?
The Point
• So we proved again that the work is the same either way (if there is no friction)
• The force you need to exert using an incline is less, but you need to exert it for a longer distance.
• Force for incline lift = mgsin()
• Force for vertical lift = mg
• Similar to how a pulley works.
However
• Typically we do not use frictionless ramps in real life, so there is more friction on the slope than in the air. Causing more work to be done on the slope.
• However, even with friction it requires less force to use ramps, and that’s why we use them.
Work Graphs
• How would you calculate the work done if you were given the following graph?

Force

Displacement

Work Graphs
• Calculate the total work done using the graph below.
Work Graphs and Calculus
• Plotted data is not always in nice straight lines!
• That’s why you need calculus…to do physics
Physics Teacher
Bellringer
• A soccer player exerts force of 37N on a soccer ball over a distance of 0.15m. How much work did the soccer player do on the soccer ball?
• W=Fd
• W=37N*0.15m
• W=5.55J
Objectives
• Practice using our new equation for work.
• Learn how time and the change of total energy of a system is related.
• Use it to solve problems
Doing Work
• If I use a constant 20N force to move a desk 2m to the right how much work have I done?
• If I then use a constant force of 20N to move a desk 2m to the left how much work have I done?
• What is the total work I have done to the desk?
Doing Work
• Unlike displacement, the total work done is not zero if I move an object around and return it to its original location.
• Work is the change in energy of a system, so once I transfer my energy to the desk to move it, the desk doesn’t transfer it back to me when I return it.
Let’s Practice
• On page 268 in the textbook answer questions 1, 2, 3 and 4
• Let me know if you have any questions
• Feel free to work with a neighbor
• Because W=Fd, doubling the force would double the work to 1.35J
• Because W=Fd, halving the distance would cut the work in half to 0.68J
• 29,000J
• 58,000J
• 600J
• 5,900J
• 110J
Real Challenge Problem
• If a 0.005kg bullet is fired at 200m/s into a 5kg block that is attached to a pendulum. The bullet gets lodged into the block, causing the pendulum to swing. How high does the pendulum swing? Ignore friction.
• 2m
Real Challenge Problem
• What is a baseball’s maximum height if it is thrown straight up with a velocity of 20m/s?
• Is this an energy problem or kinematics problem?
• Solve it both ways then tell me!
Bellringer
• A soccer player exerts force of 37N on a soccer ball over a distance of 0.15m. How much work did the soccer player do on the soccer ball?
• W=Fd
• W=37N*0.15m
• W=5.55J
Objectives
• Learn how time and the change of total energy of a system is related.
• Use it to solve problems
Energy and Time
• So far we have not related the change in energy of a system and time.
• Is it possible?
• Yup!
Stack of Books
• Is the amount of work done in the two scenarios different?
• What is different about the two scenarios?
• Which do you think is more difficult to do?
Power
• Power is equal to the change in energy divided by the time required for the change.
• The units are J/s or Watts (W)
• 1 Watt = 1 Joule of energy transferred in 1 second
Power
• It is written differently on your reference table:
Stack of Books
• Either way I did 100J of work on the stack of books. How much power did I exert if I completed the work in 2.0 seconds?
• What about if it took me 45 seconds?
• Doing work faster requires more power.
The “Variety” of Power
• A forklift can lift a 900N weight at 4 m/s. How much power is it exerting?
• Your car engine exerts a force of 5,000N over a distance of 30m in 5.0 seconds. How much power is it exerting?
Heavy Lifting
• The big guy can squat 180kg a distance of 0.8m in 2.2 seconds. The small guy can squat 85kg a distance of 1.0m in 1.2 seconds. Who is more powerful?
• Big Guy = 642 Watts
• Small Guy = 695 Watts
Try it!
• A tennis racket exerts 300 Watts of power with a 6.0N force on a tennis ball. What is the velocity of the tennis ball?
• 50m/s
• How long does it take for you to drive 50m if your engine is producing a constant force of 500N and is exerting 12,500 Watts?
• 2 seconds
Practice
• Textbook page 272, questions 10, 11a, 11b, 12, and 13.
• this may be collected and grade, hint, hint
• 1,150 W or 1.15kW
• a. 348 W b. 696W
• 0.63kW
• 130,000N
• 5.7 minutes
Real Challenge Problem
• What is a baseball’s maximum height if it is thrown straight up with a velocity of 20m/s?
• Is this an energy problem or kinematics problem?
• Solve it both ways then tell me!
Homework
• Complete the “Work and Power HWs” worksheet by Tuesday.
• It will count as two homework assignments.
• 1-15, and 16-30
• Also next week you will have a test on Energy, Work, and Power
Bellringer
• If you do 200 Joules of work on a baseball while your bat is in contact with it for 0.25 seconds, how much power was in your swing?
Objective
• Collect data and begin to calculate once and for all which one of you is the most powerful.
• Energy, Work, and Power Test will start this Thursday.
• 25 Multiple Choice
Homework
• Like the last homework I will mark the correct answers, then give it back to you so we can review it.
Lab Time
• Please take a minute to read the mission, and get in groups of three.
• At least one person from each group of three needs to compete.
• We will race just 10m, and assume that you are moving with constant acceleration.
Bellringer
• What is the power required to vertically lift a 150lbs(68kg) weightlifting bar 0.6m in 0.5seconds?
• 800 W

DO WORK

STOP

Objectives
• Review and clear up any final misconceptions, and fuzzy thinking before tomorrow’s test.
Tomorrow’s Test
• I still haven’t made it so who knows what to expect!
• Oh wait…I do!
• Energy, Springs, Work, and Power Test
• 25 Multiple Choice
Homework Review
• Who has questions about the weekend’s homework assignment on Work and Power?
Part Two Practice
• Let me know if you have any questions.
Things to Study
• All of your homework assignments
• Energy HW 1-3, Spring Textbook Assignment, Work and Power HW 1&2
• These Part Two Samples
• I’ve posted all the solutions online
Power Trip
• Who did the most work?
• Survey says!
• Who did the most power?
• Survey says!