Kinetic Energy, Potential Energy

1
Kinetic Energy, Potential Energy Efficiency

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

2
Kinetic Energy, Potential Energy Efficiency

• To understand qualitatively the concepts involved
  with K.E. G.P.E.
• To be able to successfully tackle K.E. G.P.E.
  Problems.
• To understand the concept of efficiency to
  complete efficiency calculations

Book Reference Pages 151-152
3
Kinetic Energy Concepts

• 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

4
Kinetic Energy Derivation
Object at rest (u)
Object with speed v at time t
F
m
m
F
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 s½(uv)t but u is 0 so ?
s½vt Using Newtons 2nd law... Fma actually
Fm?v/t WD Fs mv/t x ½vt ½mv2 Kinetic
Energy ½mv2 (Like all forms of energy the unit
is Joules (J))
5
Kinetic Energy implications

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

6
G.P.E. Concepts

• 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

7
G.P.E. Derivation
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))
8
Conversion between GPE KE

• 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

9
Efficiency Intro
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)

10
Efficiency Sankey
Sankey diagrams are a common way to illustrate
efficiency....
Can be applied to either energy (J) or power (W)
11
Efficiency Calculation
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 lt 1. Hence efficiency can be readily
expressed as a percentage (e.g. 0.18 18)
12
Summary
Kinetic Energy ½mv2 GPE at height h
mgh Efficiency Useful energy output Total
Energy input