Gravitation

Gravitation: Everybody attracts other body by a force called force of gravitation.

Newton’s law of Gravitation: The force of gravitational attraction between two point bodies is directly proportional to the product of their masses an inversely proportional to the square of the distance between them.

Consider tow point bodies of masses m₁ and m₂ are placed at a distance

r. the force of gravitational attraction between them, f= G ᵐ₁ ᵐ₂ /2r

Here g is constant called universal gravitational contant. The value of G is 6.67 x 10 ⁻¹¹ Nm² / kg².

Gravity: The gravitational force of earth is called gravity i.e. gravity is the force by which earth pulls a body towards its centre.

The acceleration produced in a body due to force of gravity is called acceleration due to gravity (denoted as g) and its value is 9.8 m/ s².

·         Acceleration due to gravity is independent of shape, size and mass of the body.

Variation in g

(i)                  Value of g decreases with height or depth from earth’s surface.

(ii)                G is maximum at equator.

(iii)               G is minimum at equator.

(iv)              G decreases due to rotation of earth.

(v)                G decreases if angular speed of earth increases and increases if angular speed of earth decreases.

If angular speed of earth becomes 17 times its present value, a body on the equator becomes weightless.

Weight of a body in a lift

(i)                  If lift is stationary or moving with uniform speed ‘(either upward or down ward), the apparent weight of a body is equal to its true weight.

(ii)                If lift is going up with acceleration, the apparent weight of a body is more than the true weight.

(iii)               If lift is going down with acceleration, the apparent weight of a body is less than the true weight.

(iv)              If the cord of the lift is broken, it freely. In this situation the weight of a body in the lift becomes zero. This is the situation of weightlessness.

(v)                While going down, if the acceleration of life is more than acceleration due to gravity, a body in the lift goes in contact of the ceiling of lift.