1. Unit 6
Energy, Work & Power

2. Learning Objectives You will learn
the relationship between work and energy change
kinetic energy = ½mv2
gravitational potential energy = mgh
conservation of energy

3. Work Done (J) = Energy Used (J)
Energy and Work
When we do work, we use energy.
If we do more work, we use more energy.
Hence,
Work Done (J) = Energy Used (J)
When one joule of work is done, one joule of energy is used up.
Def: Energy is the capacity to do work.
Qn: Where do we get this energy?

4. Different forms of energy
Energy- the ability to do work
Some examples of energy
Different forms of energy
Examples
Potential Energy
Elastic P.E.
Strained body: wound up spring of a clock or a stretched elastic band
Gravitational P.E.
Object placed at a height
Chemical P.E.
Substances that can be burnt such as food we eat, petrol, wood and plants
Kinetic Energy
K.E.
Object that is moving: a flying arrow or a moving bus

5. energy Kinetic energy (K.E.) K.E. = ½ mv2
Kinetic energy (K.E.) is the energy that an object has due to its motion.
Formula
K.E. = ½ mv2
Where K.E. is measured in J
m is measured in kg
v is measured in m/s
How does speed affect kinetic energy for an object?
The faster the object moves, the greater is its kinetic energy.

6. energy Kinetic energy (K.E.)
A car of mass 1000 kg is moving at a speed of 10m/s. What is its kinetic energy?
If the speed is increased to 20m/s, how many times will the kinetic energy be increased by?
Kinetic energy = ½ x m x v2
= ½ x 1000 x 10 x 10
= J
= ½ x 1000 x 20 x 20
= J
= 4 (50 000) J
= 4 times

7. gravitational potential energy at reference level is defined as zero
Gravitational potential energy (G.P.E.)
Gravitational potential energy is the energy a body possesses because of its position relative to the ground.
GPE = mgh
Where GPE is measured in J
m is measured in kg
g is a constant at 10 m/s2
h is measured in m
gravitational potential energy at reference level is defined as zero mg h
Reference level
GPE = 0 J
How does height affect GPE?
The further a body is from the ground, the greater is its gravitational potential energy.

8. Note that gravitational potential energy is never measured absolutely.
It is ALWAYS measured with respect to a certain level
This level is called reference level
we only consider the change in vertical height.

9. energy Gravitational potential energy (G.P.E.)
A car of mass 1000 kg is raised up to a height of 5m by an electromagnet. What is the gain in gravitational potential energy of the car?
Another car of weight 800 kg is raised to a height of 7m. What is the gain in gravitational potential energy of the car?
Gravitational potential energy = m x g x h
= 1000 x 10 x 5
= J
= 800 x 10 x 7
= J

10. Energy Conversion and Conservation
principle of conservation of energy
The principle of conservation of energy states
that energy cannot be created or destroyed,
but only changes from one form to another.
Which means to say energy can convert from one type to another type of energy. E.g. electrical energy from a battery into light energy from a light bulb.

11. energy conversion and conservation
conversion of energy: some examples

12. energy conversion and conservation
conversion of energy: some examples

13. energy conversion and conservation
conversion of energy
Energy can change from one form into another.
1. The pile hammer is raised to a height. It has gravitational potential energy.
2. Gravitational potential energy of the pile hammer changes into kinetic energy during the fall
3. This kinetic energy is used to drive the pile into the ground

14. energy conversion and conservation
principle of conservation of energy
Ignoring effects of friction and air resistance
all potential energy (stops for an instant)
all potential energy (stops for an instant)
all kinetic energy (greatest speed)

15. An object of mass 4kg slides down a smooth curved ramp
An object of mass 4kg slides down a smooth curved ramp. What is the speed of the parcel when it reaches the bottom?
4 m

16. By the principle of conservation of energy,
Gain in K.E. = loss in G.P.E
½ mv2 = mgh
v2 = 2gh (remove common terms i.e. m and bring ½ over)
= 2 x 10 x 4
v = √160
= 12.6 m/s (3 sig fig)
Is there any assumption that you have made?
We assume that no energy is lost to the surrounding, ie negligible friction and air resistance.

17. By now you have learnt… Forms of energy kinetic energy = ½mv2
gravitational potential energy = mgh
conservation of energy