[Science] [Physics] [GS] Gravitation, Newton’s Law, Gravity & Kepler’s Law

[Science] [Physics] [GS]

Gravitation, Newton’s Law, Gravity & Kepler’s Law

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

• Newton’s law of Gravitation : The force of gravitational attraction between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. F = (G m₁m₂)/r²

       Here G is constant called universal gravitational constant.

       The value of G is 6.67×10^-11 Nm²/Kg².

• Gravity : The gravitational force of earth is called gravity or 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 and its value is 9.8 m/s².

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

• Value of g decreases with height or depth from earth’s surface.

• g is maximum at poles.

• g is minimum at equator.

• g decreases due to rotation of earth.

• g decreases if angular speed of the earth increases.

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

• On moon, the value of g is 1/6 of the value of g at the earth.

• Kepler’s law :

• (1) All planets move around the sun in elliptical orbits, with the sun being at rest at one focus of the orbit.

• (2) The position vector of the planet with sun at the origin sweeps out equal area in equal time i.e the areal velocity of planet around the sun always remains constant.

• A consequence of this law is that the speed of the planet increases when the planet is closer to the sun and vice-versa.

• (3)The square of the period of revolution of a planet around the sun is directly proportional to the cube of mean distance of planet from the sun.

       T² α r³

• Clearly distant planets have larger period of revolution. The time period of nearest planet Mercury is 88 days where as time period of farthest planet Neptune is 165 years.

• Weight of a body in a lift :

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

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

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

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

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