Transcript L29_AS2_2008_09_KE_GPE_Efficiency

What can you remember from P3 in year 11?
•
•
•
•
Definitions
Formulas
Derived 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 151-152
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, being hit by a
slow or fast moving car?
Conceptually we can see that K.E. Is related
to both mass and speed
Object at rest (u)
F
Object with speed v at time t
m
F
m
s
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 2nd law... F=ma actually F=mv/t
WD = Fs = mv/t x ½vt = ½mv2
Kinetic Energy = ½mv2
(Like all forms of energy the unit is Joules (J))
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
•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
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
•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%)
•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. 0.18 = 18% efficient)
Kinetic Energy = ½mv2
GPE at height h = mgh
Efficiency = Useful energy output
Total Energy input