O-Level Physics Notes

1. ENERGY, WORK AND POWER!
Introduction:
Hey everyone! Hope you all are good. Today we are going to study an important chapter that is work, energy and
power. First tell me can you study, or play, or do anything if you haven’t eaten anything the entire day? Probably you
can’t. The reason is simple: because you do not have the energy to do it!
Definition:
So what is energy then? We just said we can’t do work if we don’t have energy. We have already defined it, in less
scientific terms though. Energy is basically the capacity to do work. The SI unit of energy is the Joule (J) and it is a
scalar quantity as no direction is involved in it.
And now, what is work? Work done by a constant force on an object is given by the product of the force and the
distance moved by the object in the direction of the force.
Okay, so in the above diagram, a force F is applied on the object and the object moved a distance d in the direction
of the applied force. So the work done is given by:
Work Done = Force x distance moved in the direction of force
W = F x d
where force F is in Newtons (N) and distance d is in metres (m). The SI unit of Work is Joule (J). As you have noticed
the SI unit of work and energy is the same. This is because, as we have already discussed, work can only be done
when there is energy. Hence both should have the same unit - joule.
We’ll discuss Power separately later.
Types of Energy:
1. Kinetic Energy: The most obvious form of energy is in movement: we walk, life things etc, moving objects possess
kinetic energy. Can you think of examples having kinetic energy? Umm..wind? Or waves of sea? Or your flying
football? These all possess kinetic energy.
2. Potential Energy: The stored energy is called potential energy. It exists in different forms such as chemical
potential energy which is there in the food we eat and is converted into kinetic energy when we move our bodies.
Another form is the gravitational potential energy which is due to the height of the object. For example if a ball is
thrown up, it gains gravitational potential energy and when it comes back, it’s potential energy is changed to kinetic
energy. Not that difficult or is it?
Principle of Conservation of Energy:
This principle states that energy is neither created nor destroyed. It just changes from one form to another or
transfers from one body to another but the total amount remains constant.
Like we just discussed, the food we eat contains chemical energy which is converted from solar energy of sun by the
process of photosynthesis. When we move, it is converted into kinetic energy and when we climb up a flight of stairs,
it is converted into gravitational potential energy due to height..and so on, but it never destroyed!

2. Efficiency:
Let us consider us a machine for example a power station in which fossil fuels are burnt to generate electricity. We
know by the principle of conservation of energy that the energy input must equal the energy output. But we also know
that energy output is less than energy input as energy is dissipated in the process, usually into sound and heat
energy. To summarize this:
Chemical Energy = Electric Energy + Heat and Sound Energy
(Chemical energy in fossil fuels, Electric energy of electricity generated and heat and sound energy are dissipated in
the process)
We can see that the total energy output and input are same. Though the useful energy is electric energy and the rest
is wasted. So we form a general equation:
Energy Input = Useful Energy Output + Wasted Energy Output
So now we are able to measure the efficiency of a machine by the ratio:
Efficiency = ( useful energy output / energy input ) x 100 %
Task: Google different forms of energy and the different processes which may involve these energies.
Work Done:
We have already defined Work done. Let’s make it clearer: One joule is defined as the work done by a force of one
newton which moves an object through a distance of one metre in the direction of force. So you know how much
work is one joule, right? Let’s study some formulas which related energy and work.
The kinetic energy (Ek) of a body of mass m and speed v can be found by:
Ek = ½ (mv2
)
And the gravitational potential energy (Ep) of an object of mass m at height h and gravitational pull g acting on it can
be found by:
Ek = mgh
Q1. A boy drops a marble of mass 10 g from a height of 65 m. What is speed of marble when it hits the ground? Air
resistance can be neglected and g = 10 N kg-1
.
Hint: Since air resistance is neglected, the gravitational potential energy is entirely converted into kinetic energy. Therefore, we
can assume that
Ek = Ek
½ (mv2
) = mgh
Power:
Consider two boys of equal mass, A takes 40 seconds to run 20 m while B takes 80 seconds to run the same
distance. A has more power although both did the same amount of work (i.e. running 20 metres).
Now consider that boy A has a greater mass and both cover the 20 m distance in same time, who has more power
now? Boy A has to carry a larger mass, he has done more work in running than did boy B. Since he is able to do
more work in the same time, he has more power. It’s just a bit conceptual, once you get it.
So now we can define power: it is defined as the rate of work done or the rate of energy conversion.
It is given by:
P = W / t or P = E / t

3. where W and E are work done and Energy converted respectively while t is time. Energy and Work are in Joule (J)
while time is in second (s). The SI unit of Power is Watt (W).
So from the equation, 1 W = 1 J s-1
.
Q2. A boy runs a fight of stairs of 2 m in 5 s. What is his body power is his weight is 450 N?
Task: Find more questions on the Internet regarding work, energy and power and try solving them to see if you can.
Answers:
Q1. 36 m/s
Q2. 180 W