Chapter 15 Energy

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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 Formula PE = mgh m = mass (kg) g = acceleration due to gravity (9.8 m/s 2 ) 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

Energy Conversion & Conservation 15.2

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.