Download 4.1 - Work, Power and Energy

4.1 ­ Work, Power and Energy Physics 30
Unit 4 ­ Energy and Momentum
Lesson 4.1 Outline
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Energy
Work
Power
How are work and power related?
• Examples
You will be able to:
• define energy, work and power
• solve problems involving work and power
• relate previous concepts (kinematics, force, etc.) to work/power/energy
• identify different types of energy
• state the law of conservation of energy
What is Energy?
Energy is a property of objects that can be harnessed to do "work".
Another definition is that energy is a property of objects that we cannot create or destroy, but we can change the form it takes.
The Law of Conservation of Energy
"Energy cannot be created or destroyed, but can change forms."
4.1 ­ Work, Power and Energy Types of Energy
• Kinetic
• Magnetic
• Potential
• Chemical
• Mechanical
• Nuclear
• Thermal
• Sound
• Electric
• Mass­Energy Equivalence
Converting between Forms of Energy
You listen to music on your phone.
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http://commons.wikimedia.org/wiki/File:Anatomy_of_the_Human_Ear.svg
http://commons.wikimedia.org/wiki/File:Brain_power.jpg
4.1 ­ Work, Power and Energy Converting between forms of Energy
You eat supper with your family and after you finish eating, you go for a bike ride.
You lift a weight from the floor to a spot above your head.
The Unit of Energy
We use a unit known as a Joule (J) to quantify energy. An older unit used is called the calorie and you can see this older unit remains on food labels.
1 kg m2
1 J =
s2
4.1 ­ Work, Power and Energy What is Work?
In Physics, work is different from what you and I think it is on a daily basis. For example, your work may be this package of notes while mine is to plan lessons and teach students.
In Physics, Work is the transfer of energy. When energy moves from one object to another object, work must be done in some way. When looking at the work done on an object, we need to consider all the forces acting on the object. Hence, we need the net force.
Three conditions must be met in order for work to be done:
1. A force must be exerted on the object.
2. The object must accelerate due to the force.
3. The motion must be parallel to the force. Therefore, centripetal forces do not do work because they are at right angles to the motion of the object.
Is there a song to help us remember this?
Formula for Work
Change in Energy
mass
acceleration
ΔE = W = Fd = mad
Work = Force x displacement
J = N x m
Joule = Newton­meter
**The force and displacement must be in the same plane for work to be done.
4.1 ­ Work, Power and Energy How much work is done by a 15.0 N force that is applied over a distance of 12.0 m?
Applied Force
If we look at displacement in this direction, only the y­
component of the force does work, the x­component does NO work
y­component
Ө x­component
If we look at displacement in this direction, only the x­component of the force does work, the y component does NO work
4.1 ­ Work, Power and Energy Positive work is done when the applied force and displacement are in the same direction. Energy is transferred to the object.
Example: Throwing a curling stone
Negative work is done when the applied force and displacement are in the opposite direction. This means energy is transferred away from the object.
Example: "Catching" a curling stone. (Stopping it with your broom)
It is useful to decide what the object is and what the surroundings are when doing problems involving work so that you can identify the energy transfer.
How much work is done by a 10.0 N force pulling a wagon 5.00 m horizontally if the handle makes an angle of 40.0˚ with the horizontal?
4.1 ­ Work, Power and Energy You can provide up to 75 J of energy to lift a 3 kg box a distance of 2 m upwards. How much work do you need to do in order to lift it? What's the heaviest box you can lift this distance? What is Power?
Power is the rate at which work is done. The unit for Power is the Watt (W), but this is a very small unit so we normally talk about power in terms of kW or kilowatts. We also hear of power being talked about in the unit of horsepower (hp).
kg m2
J
kg m2
s2
1 W = = =
s
s
s3
1 hp = 746 W
4.1 ­ Work, Power and Energy Fd
W
P = = = Fv t
t
Power =
Again, only the component of the force that is doing work can be used to generate power
Work
= Force x velocity
time
Pout
Power Output
Efficiency = =
Power Input
Pin
How much power is generated if a 10.0 N force displaces an object 2.00 m in 5.00 s?
4.1 ­ Work, Power and Energy A tired squirrel (with a mass of approximately 1 kg) pushes itself up by applying a force to elevate its center‐of‐mass by 5 cm. If the tired squirrel does all this work in 2 seconds, then determine its power. If the energy needed to complete this task is 0.28 W, what is the efficiency of the squirrel's muscles?
When doing a chin‐up, a physics student lifts her 42.0‐kg body a distance of 0.25 meters in 2 seconds. What is the power delivered to the student's biceps if the chemical energy to kinetic energy conversion is 50% efficient? 4.1 ­ Work, Power and Energy Attachments
Work and Power Song.flv