1. Work, Energy & Machines Chapter 9
Physical Science Unit 3
Work, Energy & Machines
Chapter 9
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2. Work 9.1 W = F (d) Work Work Equation
A quantity that measures the effects of a force acting over a distance.
Work is a result of motion in the direction of the force.
There is no work without motion.
Work Equation
Work = Force x distance
W = F (d)
No movement results in d= 0,
No work is done.
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3. Work is measured in Joules (J). Distance-
Measuring work
w = F(d)
Work is measured in Joules (J).
Distance-
means distance in the direction of the force. If a force is vertical and motion is horizontal, No work is done.
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4. Find the work done by gravity when a 2.0 kg rock falls 1.5m.
w = F (d)
What is the formula for Force(F)
F = m (g) or F = m (9.8 m/s2)
w = (m · g) (d)
w = (2.0kg · 9.8m/s2)(1.5m)
w = 29 J
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5. Negative Work???
This guy is no dumb bell even though he is doing negative work! When is his work positive/negative?
Can you think of forces that tend to do negative work?
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6. Good Question...
Ex: How much work is needed to form a human pyramid as in the picture at right?
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7. Power Power: Measurement of the rate at which work is done.
How much work is done per period of time.
Running up stairs or walking.
Both have the same amount of work done.
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8. Measuring Power
Power = work/ time
Units for Power
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9. Horsepower 1hp = 746 watts(W)
Horsepower (hp) is a commonly used unit of power.
1hp = 746 watts(W)
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10. Simple Machines 9.2 The most basic machines are simple machines.
Simple machines are divided into two families:
Lever family
Inclined plane family
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11. Can you name the six Simple Machines???
levers
pulleys
wheel and axles
screws
wedges
inclined planes
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12. How Simple Machines Work???
The basic principle is (ideally): Win = Wout
that is to say, f • D = F • d
a small force applied over a large distance yields a large force over a small distance.
Actually, the work out is always LESS than the work in…Why???
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13. Machines & Mechanical Advantage
Machines result in a multiplication of a force.
Change direction of force
Increase output force, but decrease output distance.
They do NOT allow us to do the same job with less ENERGY, rather, with less FORCE—and we waste MORE ENERGY!!! Why???
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14. Mechanical Advantage equations
PHYSICAL SCIENCE HOME PAGE
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15. Ideal MA vs. Actual MA
Some machines have an “Ideal Mech. Advantage (IMA), based on their geometry…
Levers – effort arm/resistance arm
Pulleys - # strings supporting the load
Wheel and axle – radius wheel/radius axle
Ramp – length/height
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16. Lever Family
All levers contain a fulcrum, force is transferred from one part of the arm to another.
Levers are divided into 3 classes depending on the location of the fulcrum, input force, & output force.
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17. 1st class lever
Fulcrum is in the middle of the arm. Input force acts at one end, other end applies the force.
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18. 2nd class lever
The fulcrum is at one end of the arm and the force is applied to the other end.
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19. 3rd class lever
The mechanical advantage of a third class lever is less than one.
So why use it?
Many 3rd class levers are found in the human body.
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20. So what kind of lever is it???
Just ask the question: “What is in the middle???”
The fulcrum  class 1
The load  class 2
The effort  class 3
IMA = effort arm/resistance arm
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21. Levers...
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22. Wheel and Axle Made of a lever (pulley) connected to a shaft.
When the wheel is turned the axle turns.
Screwdrivers and cranks are common wheel and axle machines.
IMA = radius wheel ÷ radius axle
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23. The Inclined Plane Family
Inclined planes multiply and redirect force.
Turns a small input force into a large output force by spreading it out over a greater distance.
IMA = length up ramp/height
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24. Wedge A screw is an inclined plane Wrapped around a cylinder.
Modified inclined plane. The ramp is being pushed rather than some other object.
A screw is an inclined plane Wrapped around a cylinder.
Jar lids operate the same way.
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25. Pulleys are Modified Levers
The middle of the pulley is like the fulcrum. The rest of the pulley is the arm.
Single fixed pulley
Can you predict the force needed to lift a 100lb load???
Single movable pulley
The output force is shared by the 2 sections of rope pulling upward.
Input force on the right only supports half the weight.
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26. Adding multiple pulleys makes a block and tackle system
Adding multiple pulleys makes a block and tackle system. It will give an even higher mechanical advantage. Can you determine the MA for each system below?
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27. Efficiency of Machines
When using a machine, not all of the work you put into the machine transfers to the object you to do work on. Why???
Friction and air resistance are two forces that dissipate energy. Where does it go???
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28. Calculating Machine Efficiency
All of the useful work put into the machine does not result in useful work done by the machine.
EFF = Wout ÷ Win
Answers are usually expressed as a % and must be less than 1 (100%). Why?
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29. Energy 9.3 Mechanical Energy -Kinetic The ability to do work.
An object has energy if it is able to produce change in itself or its surroundings.
2 Basic Forms of Energy Mechanical and non-mechanical
Mechanical Energy
-Kinetic
-Potential
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30. Nonmechanical Energy Electromagnetic (Light) Nuclear
Thermal (Heat flow)
Chemical
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31. Kinetic Energy Energy due to motion.
A brick falling at the same speed as a ping pong ball will do more damage.
KE is dependent on mass.
KE also depends on speed or velocity (v)
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32. Kinetic Energy -The unit for energy is the Joule(J)
-Energy that appears in the form of motion.
-Depends on the mass and speed of the object in motion.
-The unit for energy is the Joule(J)
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33. Potential Energy
-Energy that is a result of an objects position or condition.
-All potential energy is
Stored Energy.
i.e. Pull back on a bow string and bend the bow. The object then possesses potential energy.
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34. -This is a form of gravitational potential energy.
- A rock at the top of a ramp has more potential energy than when it is on the ground due to its position.
-This is a form of gravitational potential energy.
-Fuel is an example of chemical potential energy, due to its ability to burn.
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35. Gravitational Potential Energy
Depends on mass and height.
PE = m(g)h = W·h
m = mass
g = acceleration due to gravity
h = height
Units
m = kg
g = m/s2
h = m
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36. Conservation of Energy 9.4
In a roller coaster all of the energy for the entire ride comes from the conveyor belt that takes the cars up the first hill.
Baseball
Highest point = no kinetic energy
Lowest point = high kinetic energy
Where is the potential energy the highest?
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37. Law of Conservation of Energy
Energy can change from one form to another.
Energy can not be created or destroyed.
The total energy in the universe is constant.
Activity: Observing a Pendulum…
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38. The sum of the pendulums potential and kinetic energy is called its mechanical energy.
ME = PE + KE
ME = mechanical energy
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39. If energy is conserved, why does the pendulum eventually stop?
What happened to the mechanical energy?
When energy changes form mechanical to thermal it loses usefulness. It can not change back to mechanical by itself.
Thermal. Sound
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40. Elastic potential energy: Some forms of matter are elastic.
This means when stretched they will go back to the original shape:
Elastic potential energy is calculated with the formula: E = ½k·x2
“x” is the distance of the stretch and “k” is the “elasticity” of the material
Which has more elasticity, the steel of a car spring or a rubber band?
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41. Actually, steel is more elastic than rubber
Actually, steel is more elastic than rubber! If we use elasticity as the ability to return to it’s shape after being deformed, NOT which is easier to stretch.
Steel is able to have a greater force applied to it and still return to its original shape (e.g. greater k-constant)
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42. EPE Examples
How much energy is stored in a spring that has elasticity k=225 N/m and is stretched 20.0 cm?
What is the effective spring constant of a trampoline that is stretched m by a 200 lb person jumping up and down?
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43. What is an elastic collision?
An elastic collision is when objects collide or rebound without lasting deformation or generation of heat. Retain energy.
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44. What is an inelastic collision?
Objects collide and become distorted or generate heat. Retain less of their energy.
Clay, putty, and dough are inelastic.
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