O-Level Physics Notes

1. MASS, WEIGHT AND DENSITY!
Introduction:
Hello everybody! Hope you are all fine. Today we are going to study in detail about mass, weight and density. We are
all familiar with these quantities through our everyday lives. For example, when we ask the shopkeeper to give us a 5
kg bag of rice, we actually mean that the mass of the bag is 5 kg, and not it’s weight. So now we know that mass and
weight are two different quantities in physics, though in our everyday life we often misuse the terms. So I’m sure
none of you will find anything difficult in this. So let’s get started!
Definition:
So then what is the difference between mass and weight? Mass is the measure of amount of substance in a body,
irrespective of it’s shape, location and speed. In fact, mass depends on the number and composition of atoms and
molecules that make up the body. The SI unit of mass is kilograms and it is a scalar quantity. Coming to weight,
weight of an object is the force on the object due to gravity. It is a vector quantity and it’s SI unit is Newton (N). If it is
a vector quantity, then..what is it’s direction? We just learned that it is the pull of gravity, so definitely the direction is
downwards-towards the centre of Earth.
Density of a substance is defined as the mass per unit volume. It’s SI unit is kilogram per cubic metre kg m-3
. We’ll
study these in more detail in this tutorial.
Gravitational Field Strength:
Gravitational field is basically the region surrounding the Earth where gravity is experienced.The force gets weaker
as we move away from the Earth’s surface.
The weight of an object depends on the strength of the gravitational pull. For example, an object weighs much less
on the surface of Moon because it’s gravitational pull is much less than Earth’s pull. Gravitational Field Strength g is
defined as the gravitational force acting per unit mass on an object. On Earth, it is 9.81 N kg-1
(to make calculations
easier, we use 10N kg-1
). This means that a mass on 1 kg experiences a force of 9.81 due to Earth’s gravitational
pull. On the other hand, the same 1 kg mass experiences a gravitational force of only 1.6 N on the surface of Moon.
So the gravitational force on Moon is 1.6 N kg-1
.
We have studied in the previous tutorial on Forces the equation relating mass and weight which was derived from
Newton’s Second law of motion i.e. F = ma; and the derived equation was W = mg.
Q1. A mobile phone has a mass of 80g. Find it’s weight if g is 10N kg-1
.
What is Inertia?
Inertia of an object is the reluctance of the object to change it’s state of rest or motion. Sounds difficult? Let’s make it
easier!
Place a cardboard on an empty tumbler and a coin on the cardboard as shown in the figure.

2. Now, flick the cardboard with the finger. What do you observe? The coin drops into the tumbler.
How can we explain it in terms of inertia? When we flick the cardboard the cardboard moves fast whereas the coin
continues in its state of rest (i.e. inertia!) and hence drops into the tumbler. This tendency of an object to resist any
change in its state of rest or of uniform motion is called inertia. A bit easier now, isn’t it?
In the below diagram, the driver is saved from hitting his head in the windscreen due to seat belt. How? Without a
seat belt, if the driver suddenly applies brakes, he will continue to move forward due to his inertia and crash into the
windscreen.
Density:
Density can be given by the following formula:
ρ = m/V
where ρ is density, m is mass of object (in kg) and V is volume of object in m3
.
As we saw earlier, the SI unit of density is therefore kg m-3
.
Q2. A cube of side 2 cm has a density of 6 g cm-3
. What is the mass of the cube?

3. Answers:
Q1. 0.8 N
Q2. 48 g