What can you remember from P3 in year 11? Definitions Definitions Formulas Formulas Derived Units Derived Units Actual units Actual units

1.To understand qualitatively the concepts involved with K.E. & G.P.E. 2.To be able to successfully tackle K.E. & G.P.E. Problems. 3.To understand the concept of efficiency & to complete efficiency calculations Book Reference : Pages

Kinetic energy : the energy of an object due to movement. No movement no energy! Which would hurt the most, dropping a feather on your toe or a 1kg lump of iron? Which would hurt the most, dropping a feather on your toe or a 1kg lump of iron? Which would hurt the most, being hit by a slow or fast moving car? Which would hurt the most, being hit by a slow or fast moving car? Conceptually we can see that K.E. Is related to both mass and speed

Looking at the work done, (the energy given to the object) by force F.... (WD = Fs) If the object has speed v at time t we can find the distance with: s=½(u+v)t but u is 0 so  s=½vt Using Newton’s 2 nd law... F=ma actually F=m  v/t WD = Fs = mv/t x ½vt = ½mv 2 Kinetic Energy = ½mv 2 (Like all forms of energy the unit is Joules (J)) F F Object at rest (u)Object with speed v at time t s mm

1.No mass means no kinetic energy 2.No speed means no kinetic energy 3.Velocity is squared.... Doubling speed quadruples the KE 4.Lots of implications for stopping distances and other aspects of car safety

Gravitational Potential Energy : the energy of an object due to its position I like to think of it as the potential to fall I like to think of it as the potential to fall Relative to a “ground” level Relative to a “ground” level A form of stored energy. Hydro Electricity is a good example.... You cannot easily store excess electricity generating capacity.... Pump the water back up the hill A form of stored energy. Hydro Electricity is a good example.... You cannot easily store excess electricity generating capacity.... Pump the water back up the hill

If we raise an object of mass m, we are doing work against the weight (mg) of the object. We cannot create/destroy energy and so this work done has to go somewhere.... Work Done in raising the object = force x distance moved = mg  h GPE at height h = mgh (Like all forms of energy the unit is Joules (J))

Problems are popular where there are conversions between GPE & KE and vice versa.... Roller coasters Roller coasters Pendulums Pendulums Gain in kinetic energy is related to the loss in potential energy Take care! System maybe perfect or energy may be lost to the work done against air resistance and friction etc

Not all the energy going into a system is being used to do the intended job A tungsten filament light bulb is a worrying example. For a typical 100w bulb about 10w of useful light energy the rest is lost as unwanted heat. 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 illustrate efficiency.... Can be applied to either energy (J) or power (W)

Different ways for saying the same thing.... Efficiency = Useful energy output Total Energy input Efficiency = Work done by the machine Energy supplied to the machine This will yield a number which is nearly always < 1. Hence efficiency can be readily expressed as a percentage (e.g = 18% efficient)

Kinetic Energy = ½mv 2 GPE at height h = mgh Efficiency = Useful energy output Total Energy input