unit 07 “work, power, energy and energy conservation” introduction to work, power and energy
TRANSCRIPT
Activity Pushing against the wall
Pushing against the chair
Lifting a pailCarrying a pail
Picking up a Barbell
Holding a Barbell
Reading a book/HW
My job at McDonaldsJust think about doing this one!
Activity #1: Am I doing Work?Try each activity below and ask yourself…
“Am I doing work?”
Activity Do you think they are doing work?
(The “everyday” definition)
Does science think they are doing work?(The “physics” definition)
Pushing against the wall
Does the student do work on the wall?
Pushing against the chair
Does the student do work on the chair?
Lifting a pail Does the student do work on the pail?
Carrying a pail Does the student do work on the pail?
Picking up a Barbell
Does the student do work on the barbell?
Holding a Barbell
Does the student do work on the barbell?
Reading a book/HW
Does the student do work on the book?
My job at McDonalds
Does the student do work on McDonalds?
Yes!
Yes!
Yes!
NO!
NO!
NO!
NO!
NO!
Work is:
Equation for Work
W = FdWork = Force x distance
The 3 criteria that need to be met for work to be done on an object are:
•Force must be applied to the object
•The object must move (must be displaced)
•Force on the object and displacement of the object must be in same direction
The force applied to an object multiplied by the resulting displacement.
Units for Work
Joule (J)
Kg.m2/s2
Work is a vector:
Work includes amount and direction.
Work is – (negative) If the force pushes west, south, to the left, down …
Work is + (positive)If the force pushes east, north, to the right, up ….
Many times friction does (-) negative work on an object!You pull the box to the right: (+) work!
Friction pushes against the box to the left: (-) work!
A boy lifts a bucket of water vertically. Does he do work on the bucket?
A boy holds a bucket of water. Does he do work on the bucket.
Guide:
Is there a force on the bucket?
Does the bucket move?
Is the force and displacement in the same direction?
So does he do work on the bucket?
Yes, upYes, up
Yes, both upYes!!!!
No, the boy does not do work is done on the bucket. He does put a force on the bucket (upwards)…But the bucket does not move, no displacement…So no work!
The boy holds a bucket of water and carries it water horizontally. Does he do work on the bucket?
The boy pushes a bucket of water horizontally across the floor. Does he do work on the bucket?
No, the boy does not do work is done on the bucket. He does put a force on the bucket (upwards)…And the bucket does move (forward)…But the force and displacement are not in the same direction!So no work!
Yes, the boy does do work is done on the bucket. He does put a force on the bucket forward)…And the bucket does move (forward)…Same direction! Work is done!
Activity Does science think they are doing work?(The “physics” definition)
Pushing against the
wallDoes the student do work on the wall?
Pushing against the chair
Does the student do work on the chair?
Lifting a pail Does the student do work on the pail?
Carrying a pail Does the student do work on the pail?
Picking up a Barbell
Does the student do work on the barbell?
Holding a Barbell Does the student do work on the barbell?
Reading a book/HW Does the student do work on the book?
My job at McDonalds Does the student do work on McDonalds?
Yes! The force applied moved the barbell!
Yes! The force applied moved the chair!
Yes! The force applied moved the chair!
NO! The force and distance are not in the same direction!
NO! The wall doesn’t not move.
NO! The barbell does not move!
NO! No force applied, no distance either!
NO! No force applied, no distance either!
Efficiency
Equation for EfficiencyEff = Wout x100 Win
A perfect or ideal machine means the “Work In” would be equal to the “Work Out” since you would be the only thing doing work on an object.
But, in real life, the work in is not equal to the work out. This is because friction also does work. Friction does negative work.
Machines are not 100% efficient because of the force of friction!
Activity #2What is Power?
• Two students are both given the task of lifting a barbell over their heads (the same distance) 10 times.
• Student “B” takes 10 seconds to complete the task.• Student “S” takes 5 seconds to complete the task.
Who does more work? Who is more powerful?They do the same amount of work!
W=Fd Same force (same weight), same distance, same work!
Bobby is more powerful!!! WHY?
Equation for Power
Units for Power
Power is:
The rate at which work is done.“rate” means the time it takes.So power is how fast the work is done.
Power = Work time
P = W_ Δt
Watts (W) J/sEnglish Unit
Horsepower (hp)
746 W = 1 hp
Does power change the work done?Bobby and Sally both lift the barbell shown below. •They lift it up the same distance. •Bobby takes 10 seconds to lift the box. •Sally takes 5 seconds to lift the box.
Power ___________ change the work done!!!Work depends on
Power depends on
Who does more work? Bobby, Sally or neither?Neither! Same distance, same force! Same work!
Who has more power? Bobby, Sally or neither?Sally! Less timeMore power!
does not
Force and distance. Time does not change the work done. How fast does not change the work done.
Work and time. The faster the work is done, the more power.
• Energy: The ability to do work. • IF work is done, energy is used. If an object or person
has energy, it can do work!Units: Joule (J)
Energy has two categories:
Mechanical Energy
•Kinetic Energy•Potential Energy
Non-Mechanical Energy
•Chemical Energy•Thermal Energy•Electromagnetic Energy•Nuclear EnergyIn total there are 5 types of energy:
Mechanical, Thermal, Electromagnetic, Nuclear, Chemical
Energy
Mechanical EnergyKinetic Energy
The Energy of motionPotential Energy
The Energy of position
Electromagnetic EnergyWave energy due to magnetic and electric fields“Light waves”
MicrowavesUV wavesX-RaysVisible Light … and more!
Thermal EnergyEnergy stored in fast moving molecules which creates heatHeat Can be caused by friction
Chemical Energyenergy stored in chemical bondsFoodFossil Fuels (gas, oil)
Typesof
Energy
Nuclear EnergyEnergy stored in the nucleus of an atomFission (like the atomic bomb)Fusion (like the sun)
Thermal EnergyEnergy stored in fast moving molecules which creates heatHeat Can be caused by friction
Electromagnetic EnergyWave energy due to magnetic and electric fields“Light waves”
MicrowavesUV wavesX-RaysVisible Light … and more!
Nuclear EnergyEnergy stored in the nucleus of an atomFission (like the atomic bomb)Fusion (like the sun)
Fission Fusion
Kinetic Energy
Equation Units
KE = ½mv2
The Energy of Motion
KE = ½(mass)x(velocity)2
Joules (J)
kgm2/s2
If the object is moving it has Kinetic Energy
KE = ½ mv2
Kinetic Energy = ½ (mass)x(velocity)2