Energy
A system
A system can be described as an object or group of objects that interact with each other. When there are changes in a system there are changes in the way the energy is stored.
Types of energy
The following are types of energy stores:
- chemical
- kinetic energy
- internal (thermal)
- gravitational potential
- magnetic
- elastic potential
- nuclear
Transferring energy from one store to another
Light, sound and electricity are useful forms of energy but they cannot be classed as stores of energy. They are ways of transferring energy from one energy store to a different energy store.
can you buy a box of ‘light energy’?
Law of conservation of energy
Energy can neither be created or destroyed but only converted from one form to another.
Kinetic energy
This is the energy that a moving object has such as a moving vehicle, a projectile, an aircraft, a spacecraft or an asteroid. The kinetic energy is related to the mass and velocity of the moving object as shown in the following equation:
kinetic energy = 0.5 × mass × speed²
or in symbol form:
EK = ½mv²
Where EK = kinetic energy in joules (J)
m = mass in kilograms (Kg)
v = speed in metres per second (m/s)
When performing calculations using equations remember to work in SI units as sometimes the units can be given as multiples or sub-multiples. A simple methodology to follow is:
- write down the equation to be used
- list all known quantities next to their symbol
- list the symbol of the unknown quantity
- substitute values
- re-arrange equation to make the unknown the subject if required
EXAMPLE 1
Calculate the energy of a car moving at 30 m/s and having a mass of 1500 Kg.
EK = ½mv²
List
EK = ?
m = 1500 Kg
v = 30 m/s
Substitute
EK = ½ × 1500 × 30²
= 750 × 900
= 675, 000 J
or
= 675 kJ
EXAMPLE 2
Calculate the speed of a bus travelling through town, with a mass of 5500kg and kinetic energy of 593900 J.
EK = ½mv²
List
EK = 593900 J
m = 5500 Kg
v = ?
Substitute and re-arrange for v
v² = KE/(½m)
= 593900/(½ × 5500)
= 593900/2750
= 215.96
v = √(215.96)
= 14.70 m/s
Exercise 1
1. Calculate the kinetic energy of a car moving at 18 m/s and having a mass of 1500 Kg.
2. Calculate the velocity of a bird having a mass of 0.35 Kg and kinetic energy of 50 J.
3. Calculate the mass of an aircraft moving at a velocity of 80 m/s and kinetic energy of 900 KJ.
Gravitational potential energy
Gravitational potential energy is the amount of energy an object has because of its position above the ground, i.e. its height.
Can be shortened to ‘GPE’ or Ep
· Mass m is measured in kilograms (kg).
· Gravitational field strength g is measured in newtons per kilogram (N/kg), usually taken as 10 N/kg on Earth.
· Height h is measured in metres (m).
· Ep(GPE) is measured in joules (J).
So Ep = mgh
As the sky divers fall, in the picture below, they lose gravitational potential energy.