ENERGYENERGY

ENERGY

Basic Ideas about Energy:

•It is the property of an object that gives it the ability to cause change

•It can be neither created nor destroyed, instead it is transferred from one type to another: thermal, chemical, nuclear, electrical, etc

•Units for ENERGY are kg•m2/s2 = Joule (J)

•It takes energy to get something done• i.e, Energy is converted when Work takes place

ENERGY

POTENTIAL ENERGY

Potential Energy = the stored energy of an object as a result of the position

We are going to learn about two

types of PE…gravitational

potential energy and elastic potential

energy!

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Gravitational Potential Energy (GPE)

•This type of PE is when an object’s stored energy is the result of its vertical position (height)

•It is dependent on mass and height

•Formula is Peg = mgh

Peg = mghPeg = (75)(9.8)(4)Peg = 2,940 J

Energy is a scalar quantity, so direction is unimportant!

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EXAMPLE:

A 2 kg book sits on a bookshelf 2.1 m high. What is the potential energy of the book relative to the floor?

Peg = mghPeg = (2)(9.8)(2.1)Peg = 41 J

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Elastic Potential Energy

•Stored energy in elastic materials as the result of their stretching or compression

•Ex: springs, bungee cords, bow and arrows

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Elastic Potential Energy

F = -kx

In this equation, the force used to stretch or compress the object (F) is equal to the amount of deformation/how far stretched or compressed (x) multiplied by the spring constant (k)

PEs = ½kx2

k = spring constantx = amount of deformation

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PEs Example

The staples inside a stapler are kept in place by a spring with a relaxed length of 0.115 m. If the spring constant is 51 N/m, how much potential energy is stored in the spring when the length is stretched to 0.150 m?

PEs = ½kx2

PEs = ½(51)(0.150-0.115)2

PEs = ½(51)(0.035)2

PEs = .031 J

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Kinetic Energy

•Kinetic energy is the energy of motion

•It is dependent on mass and velocity

•Formula is KE = ½mv2

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KE Example

Determine the kinetic energy of a 625 kg rollercoaster cart that is moving with a speed of 18 m/s.

KE = ½mv2

KE = ½(625)(18)2

KE = ½(625)(324)KE = 1.05 x 105 J

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Relationship Between PE and KE

•As an object moves, the amount of PE and KE may shift back and forth

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Graphing

•In a Force (F) vs. Deformation (x) graph, the slope of the line is the Spring Constant (k)

You can reason this out since slope = y2-y1/x2-x1 (which is the same as

dividing Force by Deformation) and the

rearranged equation for F = -kx would then be k

= F/x

Spring Constant (k)