Chapter 6 Work and Energy 6.1 – Work Work Formula & Units Positive & Negative Work 6.2 –...
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Transcript of Chapter 6 Work and Energy 6.1 – Work Work Formula & Units Positive & Negative Work 6.2 –...
![Page 1: Chapter 6 Work and Energy 6.1 – Work Work Formula & Units Positive & Negative Work 6.2 – Work-Energy Theorem & Kinetic Energy KE Formula & Units 6.3 –](https://reader033.fdocuments.us/reader033/viewer/2022061418/56649f2c5503460f94c47492/html5/thumbnails/1.jpg)
Chapter 6Work and Energy6.1 – Work
Work Formula & UnitsPositive & Negative Work
6.2 – Work-Energy Theorem & Kinetic EnergyKE Formula & Units
6.3 – Gravitational Potential EnergyGPE FormulaPositive & Negative Work
6.4 – Conservation of EnergyTotal Mechanical Energy
6.5 – PowerPower Formula
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Work is done on an object whenever a force is applied parallel to the displacement.
6.1 – Work Done by a Constant Force
Work = Force x Displacement
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Less work is done on the object in bottom figure.
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( cos )W F s
displacement (m) force (N) work
(N·m or Joule)
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( cos )W F s
θ = 0°; cosθ =1W = F(s)
θ = 90°; cosθ = 0W = 0
θ = 180°; cosθ = -1W = - F(s)
θ = 270°; cosθ = 0W = 0
Block is moving this way
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Person is doing positive work
on the barbell when lifting.
Person is doing negative work
on the barbell when lowering
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Work can be positive or negative, but it is NOT a vector.
Work is measured in Joules (Newton-meters) or ft-lbs
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1. Lifting a weight up off the floor.
Are you doing work on the object?
2. Pushing a truck as hard as you can but the truck doesn’t move
3. Carrying books across a room.
4. Lowering a barbell during a bench-press rep.
5. Gravity pulling a ball down to earth.
6. Gravity pulling on a book resting a table.
YES
YES, negative work
YES
NO
NO
NO
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For now, a good way to know if work is done is to see if the PE or KE of the
object is changed.
Work will cause a change in energy of the object.
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Ch. 6 Homework #1
Ch. 6Problems #1-5 (p. 180)
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Energy - The ability to do work; measured in Joules
Kinetic Energy - Energy due to motion
21
2KE mv
mass (kg)
velocity(m/s)
6.2 – Work-Energy Theorem & KE
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F ma Fd W mad
2 20 2fv v ad
2 20
2fv v
ad 2 2
0
2fv v
W m
2 21 102 2fW mv mv
f iW KE KE
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The Work-Energy Theorem -
A net external force on an object changes the KE of the object.
The change in KE of the object equals the work that was done on the object
W = ΔKE
f iW KE KE
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Ch. 6 Homework #2
Ch. 6Problems #12,13,15,17
p. 181
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Potential Energy -
Energy due to relative position
Elastic Potential EnergyElectrical Potential Energy
Gravitational Potential Energy
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6. 3 - Gravitational Potential Energy
Work done by the force of gravity
0( cos 0 )( )grav fW mg h h
( cos )W F s
gravW mgh
height difference (m)
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Gravitational Potential Energy
height (m)
PE mgh
The work done by gravity does not depend on the path taken, only the
height difference.
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The total mechanical energy (E) of an object remains constant, neglecting frictional forces.
E = KE + PE
6. 4 – Conservation of Mechanical Energy
Einitial = Efinal
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The Kingda Ka is a giant roller coaster with a vertical drop of 127 m. Suppose that the coaster has a speed of 6.0 m/s at the top of the drop. Neglect friction and air resistance and find the speed of the riders at the bottom in miles/hour
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Chapter 6 Homework #3
Ch. 6Problems #25,26,28,35,32,36page 182
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Power - the rate at which work is done.
1 horsepower = 550 ft-lbs/sec = 745.7 watts
(joules) (watts) =
(sec)
WorkAverage Power
time
6. 5 – Power
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Conservation of Energy Lab
When block is moving up or down at constant velocity, the net force is zero.
Fup = Fgrav + fk Fdown = Fgrav - fk
Fup + Fdown = 2 (Fgrav )
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Conservation of Energy Lab
4. Work = Fgrav x length
1. W = mg
2. Fgrav = (Fup + Fdown) /2
3. Fgrav = Wsinθ
5. ΔPE = mgh
6. Workactual = Fup x length
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Ch. 6 Equations
( cos )W F s21
2KE mv
f iW KE KE
gravW mgh (joules)
(watts) = (sec)
WorkAverage Power
time
E = KE + PE
Einitial = Efinal