The Nature of Energy

Energy

The ability to cause change or the ability to do work

Joule – the SI unit used to measure energy

Energy Two main types of energy:

Kinetic Potential

Other types of energy include… Thermal Light Sound Electrical Chemical Nuclear

Kinetic energy

Energy in the form of motionKinetic energy = ½ x mass x velocity2

KE = ½mv2

Work - Energy Theorem Net work = change in KE Wnet = ΔKE (“change in” means final minus initial)Wnet = KEf – KEi

When you combine the kinetic energy formula and the work energy theorem…you get

Wnet = (½ mv2)f – (½ mv2)i

Potential Energy

Energy stored in a motionless object, giving it the potential to cause change

3 Types of Potential Energy Elastic Potential Energy – Energy stored by

things that stretch or compress (springs, elastic, rubberbands)

Chemical Potential Energy - energy stored in chemical bonds between atoms (Snickers bar, food, even gasoline)

Gravitational Potential Energy – energy stored by things that are above Earth (book sitting on shelf, person standing on a cliff, rollercoaster perched at the top of a hill)

Gravitational Potential Energy

GPE = mass x acceleration due to gravity x height

GPE = mgh

Be aware that U is the general variable used for all types of potential energy in upper level physics…different types are denoted by subscripts on the U.

Ug = mgh

The energy an object has because of its motion or position; The sum of the kinetic energy and all forms of potential energy in a system; for now we will focus only on GPE

ME = KE +∑PE (real formula) ME – KE + GPE (current use of formula)

NOTE: This is NOT the total energy. Remember that other types also exist in the world… Thermal Light Sound Electrical Chemical Nuclear

Mechanical Energy

Law of Conservation of Energy

Energy may change from one form to another, but the total amount of energy in a system does not change

MEi = MEf or…

KEi + PEi = KEf + PEf

Law of Conservation of Energy (example)

Fuels store energy in the form of chemical potential energy.

This energy is transformed into heat energy, which is then transformed into kinetic energy.

Ex: 1…gasoline put into your car (chem. Pot. Energy)…it is heated/combusted and a chemical reaction occurs so that the energy stored in the bonds can be released in the form of heat. Then, the heat energy is used to move parts like pistons, etc (kinetic energy) …which eventually moves your car!

Ex: 2…Candy bar has chemical potential energy…you eat it…your body digests it, meaning it breaks down the food – even breaking the bonds and releasing energy. Your body uses the energy to move (kinetic energy)!

Example Problem 1

A 50 kg boy and his 100 kg father went jogging. Both ran at a rate of 5 m/s. Who had more kinetic energy? Show your work and explain.

Example Problem - answer

KE = ½mv2

Boy…

KE = ½(50 kg)(5 m/s)2

KE = 625 JDad…

KE = ½(100 kg)(5 m/s)2

KE = 1250 J

Dad had more Kinetic energy because his mass was greater.

Example Problem 2

What is the potential energy of a 10 N book that is placed on a shelf that is 2.5 meters high?

Example Problem 2 - answer

GPE = mghGPE = (10 N) (2.5m)GPE = 25 JRemember that weight = mg

and that the force provided is weight.

NOTE: you may want to change your variable for weight to Fg.

Rollercoaster Example…