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Gravitation: It’s the force that acts between two different bodies and attract each other.
This law states that every object in the universe attracts every other object. The gravitational force between two objects is directly proportional to their masses and inversely proportional to the square of the distance between them.
Suppose, two objects A and B having masses M and m are at a distance of d and the force acting between them is F.
According to the universal law of gravitation, the force acting between two objects is directly proportional to their masses i.e
F ∝ M x m
The force is also inversely proportional to the square of the distance between them.
So, F ∝ 1/d2
From (1) and (2)
=> F ∝ (M x m) / d2
=> F = G (M x m) / d2
Here G is constant and is called the universal law of gravitation
=> F × d2 = G M × m
=> G = (F × d2) / (M × m)
The S.I unit of G is N m2 kg-2
Universal law of gravitation is important for the following reasons:
Free fall: When an object falls due to the gravitational force, alone it is called free fall.
From second law of motion we know that force is equal to the product of mass and acceleration.
i.e. F = m x a
Here, gravity is acting as acceleration
So, F = m x g
Now, replacing the value of F with G (M x m) / d2
=> m x g = G (M x m) / d2
=> g = G x M / d2
Here, M is the mass of earth and d is the distance between object and the earth.
Mass
Mass is the actual quantity of matter of an object. Mass of an object also defines it’s inertia. The S.I. unit of mass is Kg.
Weight
Weight is the product of mass of object and gravity. The S.I. unit of weight is kg.
The formula for weight is W = m x g
Thrust: It the perpendicular force acting on an object.
Pressure: Thrust per unit area is called pressure. The formula of pressure is:
Pressure = thrust / area
The S.I. unit of pressure is N/m2 pascal.
Buoyant force
The upward force exerted by the water on an object is called buoyant force. When an object is immersed in water buoyant force acts on it. The magnitude of buoyant force depends on the density of object.
Archimedes principle states that: when a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.
The concept of Archimedes principle is used in designing ship and submarines.
When the density of an object is less than the density of water it floats on it.
When the density of an object is more than the density of water it sinks in it.
Relative density
Relative density of any object is the ratio of its density to that of water.
The formula for relative density is:
Relative density = Density of substance/ Density of water