Energy Work Power

Textbook Chp 6 pg 103-122

What is Energy?OEnergy is the ability to do work

OUnit: Joules (J)

5 forms of EnergyO3 types of Potential Energy

OElastic Potential EnergyOChemical Potential EnergyOGravitational Potential Energy

OKinetic EnergyOThermal EnergyO[All have unit of Joules!]

Potential EnergyOPotential Energy refers to

energy which is stored to do work later

Othe energy is not doing anything now, but has the potential to do something later

OThere are 3 types of potential energy in your syllabus: elastic, potential and chemical

Elastic Potential Energy

OElastic Potential Energy is the energy stored when an elastic object is stretched or squeezed.

OE.g. a stretched rubber band

Chemical Potential Energy

OChemical potential energy is the stored energy which can only be released by chemical reactions

OE.g. battery, food

Gravitational Potential Energy (GPE)

OGPE is the energy stored due to the height of the object

Kinetic Energy (KE)OKE refers to the energy of moving objects

Thermal EnergyOThermal energy is energy

due to heat. OHot objects have high

thermal energyOThermal energy is also

created when a moving object is opposed by friction

5 forms of EnergyO3 types of Potential Energy

OElastic Potential EnergyOChemical Potential EnergyOGravitational Potential Energy

OKinetic EnergyOThermal Energy

Principle of Conservation of Energy (CoE)

OEnergy cannot be created or destroyed, but can be converted from one form to another

OThe total energy of an isolated system is constant

Conversion of EnergyOSince CoE states that energy

cannot be created or destroyed,Owhen one kind of type is lost, it

must be converted to another type of energy

OE.g. if Energy A is being converted to Energy B, we say the Loss in Energy A = Gain in Energy B

Example 1OAn object falling from a tall

height, ignore air resistanceOGravitation Potential Energy

(GPE) is lostOKinetic Energy (KE) is gainedOloss in GPE = gain in KE

Example 2OAn object sliding across a flat

rough surfaceOObject becomes slower – KE is lostOWhere is the energy converted to?OMotion against Friction always

results in gain in Thermal EnergyOLoss in KE = Gain in Thermal

Energy

Example 3OStretched rubber band is

releasedOStretched rubber band has

elastic potential energyOAfter it is released, elastic

potential energy is lostO It starts to move = kinetic

energy is gainedOLoss in Elastic PE = Gain in

KE

Example 4OAn electric toy car running on

batteriesOToy Car starts to move when

switched onO gain in KE

OBatteries lose energy over timeO loss in Chemical PE

OLoss in Chemical PE = Gain in KE

Skate Park Demo

KE and GPEOKE = ½ mv2

Om = mass of objectOv = velocity of object

OGPE = mghOm = mass of objectOg = gravitational field strength,

10 ms-2

Oh= height above the ground

Common Mistake!!OFor energy equations, the right

side of the equation MUST be in SI Units

OE.g. for GPE = mghOUnits of m must be in kg (not g)OUnits of h must be in m (not

cm)

Example 5aO An object of mass 5 kg is dropped

from rest. Determine its loss in gravitational potential energy after it has fallen 10m. Ignore air resistance.

O Loss in GPE = mghO = (5)(10)(10)O = 500 J (3 sf)

Example 5bO What is the KE gained by the object

after it has fallen for 10 m?

O Loss in GPE = Gain in KEO Loss in GPE = 500 JO Gain in KE = 500 J

Example 5cO What is the velocity of the object

after it has fallen for 10 m?

O KE = 500 JO ½ mv2 = 500O (0.5)(5)v2 = 500O v2 = 200O v = 14.1 ms-1 (3 sf)

Work DoneO In Physics, work done is to describe

when a force has moved an objectOEquation:OWork Done = Force x Distance

O Distance must be same direction as Force

OSI Units for Work Done is Joules (J)

Example 6O A force of 5N pushes an object a

distance of 10 m. Determine the work done on the object by the force.

O Work Done = Force x DistanceO = (5)(10)O = 50.0 J (3 sf)

F

10 m

Example 7aO Lionel exerts 20 N of force holding a

stack of books on his hands. He walks forward for 5 m. What is his work done?

O Ans: zero. force is not in same direction as the distance moved

Example 7bO Still holding the books, Lionel walks

up a flight of stairs to a height of 4 m. What is his work done?

O Is distance same direction as force? Yes

O Work Done = Force x DistanceO = (20)(4) = 80.0 J (3 sf)

PowerO A person/machine is said to be

powerful when he can do a lot of work in very little time

O If Zhiyu can study 3 chapters of Physics in 1 hour but Andre can study 4 chapters of Physics in 1 hour, who is the more powerful Physics mugger?

O If Zhiyu studied for 2 hours and Andre studied for 1 hour, who did more work?

PowerO There are two possible equations for

Power:O Power = Work Done / timeO Power = Energy Spent / timeO Units of Power: Watts (W)

Example 8O A force of 5N pushes an object a distance

of 10 m for a time of 5 s. Determine the power created by the force.

O Work Done = Force x Distance = (5)(10) = 50 J

O Power = Work Done / TimeO = 50/5 = 10.0 W (3 sf)

Example 9O An electric heater used 3600 J of

energy in 30 s. What is the power of the heater?

O Power = Energy spent / timeO = 3600/30O = 120 W

RecapO 5 types of EnergyO Principle of Conservation of EnergyO Conversion of EnergyO GPE = mghO KE = ½ mv2

O Work Done = Force x DistanceO 2 Eqns for Power

Quiz!!