Topic: Weight, Mass, Density

Lesson Notes - Weight, Mass

Definition: Weight is the gravitational force acting on an object.
SI unit: Newton (N)

Definition: Mass is a measure of the amount of matter in a body.
SI unit: Kilogram (kg)

Definition: The gravitational field strength at any point in a gravitational field is the gravitational force per unit mass exerted on any bodies.
SI unit: G

What is gravitational field?
A gravitational field is a region in which a mass experiences a force due to gravitational attraction.


Formula for Weight & Mass:
Weight = mass x gravitational field strength
W = mg
Formula for Gravitational field strength:
Gravitational field strength = Weight / Mass
G = N / kg

Mass
-The mass of an object is constant. It does not change even when the object is on the Moon surface in which the gravitational pull on the object changes.

-The mass of an object is a measure of the inertia of the object.

-The inertia of an object is its resistance to change in its state of rest or motion.

-Hence the larger the mass of the object, the greater is its inertia.

-Below are situations which can be explained using the idea of inertia the resistance to change in the state of rest or motion of an object.
(a) When a car suddenly speeds off, its passenger is pushed back against the seat.
- Initially the passenger is at rest.
- When the car suddenly speeds off, the passenger tends to remain in a state of rest.
- He is pushed by the seat.
(b) A passenger in a fast moving car is thrown forward when the car suddenly brakes.
- Initially the passenger is moving.
- When the car suddenly stops, the passenger tends to continue moving because of his
inertia.
- Hence he is thrown forward.


Gravitational field and field strength
- When a ball is released, it drops to the floor due to the gravitational pull of the Earth.
- There is a gravitational field around the Earth. A gravitational field is the region within which a mass experiences a force of attraction.
- The gravitational field strength g is the gravitational force per unit mass.
- On the Earth surface, the gravitational field strength g is approximately 10N/kg
- The gravitational force on a mass is known as its weight. Hence the weight of a mass m is given by (FORMULA)
- The gravitational field strength of the Earth decreases as the altitude increases.
- On the surface of the Moon, the gravitational field strength is about 1/6 the value of g on the Earth surface.
- Hence the weight W (= mg) of an object is not constant but changes when the value of g changes.
- The acceleration of an object that falls freely under the action of gravitational force is known as the acceleration due to gravity g.


Density

Definition: The density of a substance is defined as its mass per unit volume.
SI unit: kilogram per cubic metre (kg / m3)

Formula for Density:
Density = Mass / Volume
p = m / V