EnergyEnergy

January 2015January 2015

ArsenieArsenie

Two groups of EnergyTwo groups of Energy Potential Energy (PE)Potential Energy (PE)

It isn’t shown through movementIt isn’t shown through movement Energy is stored and ready to do workEnergy is stored and ready to do work

Elastic PE – a spring, a rubberband, a bowElastic PE – a spring, a rubberband, a bow Chemical PE – batteries, gasolineChemical PE – batteries, gasoline Nuclear PE – the sun, bombsNuclear PE – the sun, bombs Gravitational PE – any object elevatedGravitational PE – any object elevated

Kinetic Energy (KE)Kinetic Energy (KE) Object is in motion Object is in motion ENERGY of MOTIONENERGY of MOTION

What is What is ““MechanicalMechanical”” Energy?Energy?

Mechanical energy refers to objects Mechanical energy refers to objects in motion or that have the potential in motion or that have the potential to move. It is the sum of potential to move. It is the sum of potential and kinetic energy. and kinetic energy.

Mechanical Energy is the property Mechanical Energy is the property of an object that enables it to do of an object that enables it to do ‘‘workwork’’. .

Work Work is the product of the force (F) on is the product of the force (F) on an object and the distance (d) that an object and the distance (d) that object object movedmoved (in the same direction as (in the same direction as the force).the force).

Work is done to either: to change the speed of an object or against another existing force, such as lifting against gravity.

Work is the Energy of a ForceWork is the Energy of a Force

FdW

SI Metric Units of EnergySI Metric Units of Energy

Energy is measured in Joules (J). This Energy is measured in Joules (J). This includes Work, Potential Energy, or Kinetic includes Work, Potential Energy, or Kinetic Energy. Energy.

1 Joule = 1 Newton x 1 meter1 Joule = 1 Newton x 1 meter

1 Newton x 1 meter is also called a 1 Newton x 1 meter is also called a ““Newton-Newton-metermeter””. .

think!think!You lift a 100-N boulder 2m.You lift a 100-N boulder 2m.

Q. Q. How much work is done on the boulder?How much work is done on the boulder?

Answer:Answer:

WW = = FdFd = 100 N x 2 m = 200 N·m = 200 J = 100 N x 2 m = 200 N·m = 200 J

Example problem

Gravity exerts a force on this boulder. In order to move it, work Gravity exerts a force on this boulder. In order to move it, work must be done (against the existing force of gravity). Which must be done (against the existing force of gravity). Which example requires more example requires more workwork??

• The boulder isThe boulder is lifted with 100 N of force.lifted with 100 N of force.• The boulder isThe boulder is pushed up the 4-m incline with 50 N of pushed up the 4-m incline with 50 N of

force.force.• The boulder isThe boulder is lifted with 100 N of force up each 0.5-m lifted with 100 N of force up each 0.5-m

stair. stair.

Potential Energy:

Trick question! Each example requires the same amount of work.

Potential EnergyPotential Energy

But, itBut, it’’s also equal and opposite to s also equal and opposite to Work!!!!!!Work!!!!!! Because we would have to do work against gravity to Because we would have to do work against gravity to

lift something to get it elevated to store PElift something to get it elevated to store PE Therefore, PE converts to Work! Therefore, PE converts to Work!

So, wSo, what is the gravitational potential energy of hat is the gravitational potential energy of the boulder in the lifted position? the boulder in the lifted position?

Answer: it is still 200 J!Answer: it is still 200 J!

Why? Because PE is equal and opposite to work!Why? Because PE is equal and opposite to work!

mghPE

FdW

maF

hd So,

FhW Then,

ga

and

and So,

mgF

PEmghW Therefore,

W is equal and opposite to PE

Kinetic EnergyKinetic Energy

But itBut it’’s ALSO equal to the work required to s ALSO equal to the work required to bring something to its final motion or to bring something to its final motion or to rest because it is a conversion of potential rest because it is a conversion of potential energy.energy.

……and potential energy is also equal to work and potential energy is also equal to work and measured in Joules, and work is equal and measured in Joules, and work is equal to force multiplied by distance. Therefore:to force multiplied by distance. Therefore:

221mvKE

221mvKEPEmghFdW

The Law of Conservation of The Law of Conservation of EnergyEnergy

We have discovered a natural law of the We have discovered a natural law of the

universe that universe that all energy is all energy is conservedconserved. .

Energy is not created or destroyed. Energy is not created or destroyed. All the energy that was here in the beginning All the energy that was here in the beginning

is still here, and will be here until the end. is still here, and will be here until the end. Energy simply converts from one form to Energy simply converts from one form to

another. For example, PE converts to KE. another. For example, PE converts to KE.

KEPE

Forms of EnergyForms of Energy

ThermalThermal ChemicalChemical ElectricElectric Radiant Radiant

(electromagnetic)(electromagnetic) NuclearNuclear MagneticMagnetic

ElasticElastic SoundSound MechanicalMechanical LuminousLuminous Mass (E=mcMass (E=mc22)) Dark Dark

(theoretical)(theoretical) Nutritional/foodNutritional/food

Can you categorize each of these as Potential or Kinetic?

Forms of Energy Forms of Energy Thermal (KE)Thermal (KE) Chemical (PE)Chemical (PE) Electric (KE)Electric (KE) Radiant (KE) Radiant (KE)

(electromagnetic)(electromagnetic) Nuclear (PE)Nuclear (PE) Magnetic (KE)Magnetic (KE) Nutritional/food Nutritional/food

(PE) (PE)

Elastic (KE)Elastic (KE) Sound (KE)Sound (KE) Mechanical (PE)Mechanical (PE) Luminous (KE)Luminous (KE) Mass (E=mcMass (E=mc22) )

(KE)(KE) Gravitational (KE)Gravitational (KE) Dark (theoretical)Dark (theoretical)