What can you remember from P3 in year 11?What can you remember from P3 in year 11?

•DefinitionsDefinitions•FormulasFormulas•Derived UnitsDerived Units•Actual unitsActual units

1.1. To understand qualitatively the concepts To understand qualitatively the concepts involved with K.E. & G.P.E.involved with K.E. & G.P.E.

2.2. To be able to successfully tackle K.E. & To be able to successfully tackle K.E. & G.P.E. Problems.G.P.E. Problems.

3.3. To understand the concept of efficiency To understand the concept of efficiency & to complete efficiency calculations& to complete efficiency calculations

Book Reference : Pages 151-152Book Reference : Pages 151-152

Kinetic energyKinetic energy : the energy of an object due : the energy of an object due to movement. No movement no energy!to movement. No movement no energy!

•Which would hurt the most, dropping a Which would hurt the most, dropping a feather on your toe or a 1kg lump of iron?feather on your toe or a 1kg lump of iron?

•Which would hurt the most, being hit by a Which would hurt the most, being hit by a slow or fast moving car?slow or fast moving car?

Conceptually we can see that K.E. Is related Conceptually we can see that K.E. Is related to both mass and speedto both mass and speed

Looking at the work done, (the energy given to the Looking at the work done, (the energy given to the object) by force F.... (WD = Fs)object) by force F.... (WD = Fs)

If the object has speed v at time t we can find the If the object has speed v at time t we can find the distance with: distance with: s=½(u+v)t s=½(u+v)t but u is 0 so but u is 0 so s=½vts=½vt

Using Newton’s 2Using Newton’s 2ndnd law... law... F=ma F=ma actuallyactually F=m F=mv/tv/t

WD = Fs = mv/t x WD = Fs = mv/t x ½vt = ½mv½vt = ½mv22

Kinetic Energy = Kinetic Energy = ½mv½mv2 2

(Like all forms of energy the unit is Joules (J))(Like all forms of energy the unit is Joules (J))

F F

Object at rest (u) Object with speed v at time t

s

m m

1.1. No mass means no kinetic energyNo mass means no kinetic energy

2.2. No speed means no kinetic energyNo speed means no kinetic energy

3.3. Velocity is squared.... Doubling speed Velocity is squared.... Doubling speed quadruples the KEquadruples the KE

4.4. Lots of implications for stopping Lots of implications for stopping distances and other aspects of car distances and other aspects of car safetysafety

Gravitational Potential EnergyGravitational Potential Energy : the energy : the energy of an object due to its positionof an object due to its position

•I like to think of it as the potential to fallI like to think of it as the potential to fall

•Relative to a Relative to a “ground” “ground” levellevel

•A form of A form of stored energystored energy. Hydro Electricity is . Hydro Electricity is a good example.... You cannot easily store a good example.... You cannot easily store excess electricity generating capacity.... excess electricity generating capacity.... Pump the water back Pump the water back upup the hill the hill

If we raise an object of mass m, we are doing If we raise an object of mass m, we are doing workwork against the weight (mg) of the object. against the weight (mg) of the object.

We We cannotcannot create/destroy energy and so this create/destroy energy and so this work done work done has to go somewhere....has to go somewhere....

Work Done in raising the object = force x Work Done in raising the object = force x distance moved = mgdistance moved = mghh

GPE at height h = mghGPE at height h = mgh

(Like all forms of energy the unit is Joules (J))(Like all forms of energy the unit is Joules (J))

Problems are popular where there are Problems are popular where there are conversions between GPE & KE and vice conversions between GPE & KE and vice versa....versa....

•Roller coastersRoller coasters•PendulumsPendulums

Gain in kinetic energy is related to the loss in Gain in kinetic energy is related to the loss in potential energypotential energy

Take care! System maybe perfect or energy Take care! System maybe perfect or energy may be lost to the work done against air may be lost to the work done against air resistance and friction etcresistance and friction etc

Not all the energy going into a system is being Not all the energy going into a system is being used to do the intended jobused to do the intended job

A tungsten filament light bulb is a A tungsten filament light bulb is a worrying example. For a typical 100w worrying example. For a typical 100w bulb about 10w of useful light energy bulb about 10w of useful light energy the rest is lost as unwanted heat.the rest is lost as unwanted heat.

Other examples include:Other examples include:

•Car engine (60%)Car engine (60%)

•Standard central heating boiler (80%)Standard central heating boiler (80%)

Sankey diagrams are a common way to Sankey diagrams are a common way to illustrate efficiency....illustrate efficiency....

Can be applied Can be applied to either energy to either energy (J) or power (W)(J) or power (W)

Different ways for saying the same thing....Different ways for saying the same thing....

Efficiency = Efficiency = Useful energy outputUseful energy output

Total Energy inputTotal Energy input

Efficiency = Efficiency = Work done by the machineWork done by the machine

Energy supplied to the machineEnergy supplied to the machine

This will yield a number which is nearly always This will yield a number which is nearly always < 1. Hence efficiency can be readily expressed < 1. Hence efficiency can be readily expressed as a percentage (e.g. 0.18 = 18% efficient)as a percentage (e.g. 0.18 = 18% efficient)

Kinetic Energy = ½mvKinetic Energy = ½mv22

GPE at height h = mghGPE at height h = mgh

Efficiency = Efficiency = Useful energy outputUseful energy output

Total Energy inputTotal Energy input