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

The story of a match…

Striking a match

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.