chapter 15
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Chapter 15Energy
Windup Toy
• http://www.youtube.com/watch?v=xwCUzYuiTdk
Energy & Its Forms
15.1
Avalanche Questions
• Where did the energy of the avalanche come from?
• Where did it go?
Definitions
• Work • Done when a force moves an object
through a distance• A transfer of energy
• Energy• The ability to do work• Energy is transferred by a force moving
an object through a distance
Measuring Energy
• SI Unit of Energy• Joules (J)• 1 Joule = 1 Newton-meter
• Newton-meter• The work done when an object is
moved 1 meter by 1-Newton force
Two General Types of Energy
Kinetic Energy• Energy of
Motion
• Depends on mass & speed
• Formula
Potential Energy
• Energy that is stored as a result of position or shape
• Energy with the potential to do work
• Examples: • Lifting a book in
the air• Plucking a string
Parts of KE Formula
• K = Kinetic Energy (measured in Joules)
• m = mass (measured in kilograms)
• v = speed (measured in meters per second)
Why would tripling the speed at which a car is moving have a greater effect on its kinetic energy than tripling its mass?
KE Practice Problems
1. A 0.10 kg bird is flying at a constant speed of 8.0 m/s. What is the birds kinetic energy?
2. A 70.0 kg man is walking at a speed o 2.0 m/s. What is his kinetic energy?
3. A 1400 kg car is moving at a speed of 25 m/s. How much kinetic energy does the car have?
4. A 50.0 kg cheetah has a KE of 18,000 Joules. How fast is the cheetah running?
Gravitational Potential Energy
Gravitational PE
• PE that depends upon an object’s height
• Increases as an object is raised to a higher level
• Depends on an objects mass, height, and acceleration due to gravity
FormulaPE = mgh
• m = mass (kg)
• g = acceleration due to gravity (9.8 m/s2)
• h = height (meters)
Gravitational PE Practice Problems
#1• A diver is at
the top of a 10.0 meter-high diving platform and has a mass of 50.0 kg. What is her gravitational PE?
#2• The same
diver is standing on the ground. What is her gravitational PE? Why?
Elastic Potential Energy
• The potential energy of an object that is stretched or compressed
• Elastic means something springs back to its original shape after being stretched or compressed
• Rubber bands…
• Investigating Elastic PE Lab (p450)
Forms of Energy
• Mechanical Energy
• Thermal Energy
• Chemical Energy
• Electrical Energy
• Electromagnetic Energy
• Nuclear Energy
Mechanical Energy
• Energy associated with the motion and position of everyday objects
• Equal to the sum of KE and PE
Thermal Energy
• Matter is made of atoms that are in constant motion
• The potential and kinetic energy of all of the atomic particles in an object make up its thermal energy
Chemical Energy
• The energy stored in chemical bonds
• When bonds are broken, energy is released
Electrical Energy
• Energy associated with electrical charges
Electromagnetic Energy
• A form of energy that travels through space in the form of waves
• The sun emits electromagnetic energy
• Capable of traveling long distances in air and space
Nuclear Energy
• The energy stored in atomic nuclei
• Nuclear energy can be used to generate electricity
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Energy Conversion
& Conservatio
n15.2
An energy conversion story…
• Meteor story, p 453 (read as class)
Energy Conversion
• Energy can be converted from one form to another
• Energy conversion is the process of this change
Conservation of Energy
• Law of Conservation of Energy• Energy cannot be created nor
destroyed
• Energy changes from one form to another, but the total amount of energy remains unchanged
An example…
• You are riding your bike, pedaling at a constant rate, but then you stop pedaling and coast. You eventually come to a stop. Where did your kinetic energy go?
PE to KE
• Gravitational PE is converted to KE as an object falls to the ground
• Elastic PE to KE… Slinky TIME!
Energy story of a hungry sea gull…
• Write a quick story about how a sea gull could crack open an oyster shell.
• Energy terms to consider: Gravitational PE, KE, Chemical Energy (in the form of Gull food)
Conversion Calculations
• Mechanical Energy = PE + KE
• Can be applied to any mechanical process (which is any action, for example, a sea gull flying an oyster high into the air and then dropping it on a rock)
• In these calculations we assume friction is negligible
Conservation of Mechanical Energy
(KE + PE)Beginning = (KE + PE)End
Conservation Example
• At a construction site, a 1.50-kg brick is dropped from rest and hits the ground at a speed of 26.0 m/s. Assuming air resistance can be ignored, calculate the gravitational potential energy of the brick before it was dropped. At what height was the brick dropped from.
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