**Law of Universal Gravitation**

**Every object attracts every other object with an attractive force which is directly proportional to the masses and inversely proportional to the square of the separation between them**

*** F** = gravitational force(N)

* m** = mass of object(kg)

- r** = separation between objects(m)

*** G** = universal gravitation constant

++ **F=-GMm/r^2 g=-GM/r^2**

**From Newton’s 2nd Law**

**F = mg**= acceleration due to gravity (m/s2)

**Kepler's three laws of planetary motion are:**

- The orbit of every planet is an ellipse with the sun at a focus.

- A line joining a planet and the sun sweeps out equal areas during equal intervals of time.

- The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

**NEWTONS THIRD LAW OF MOTION AND
GRAVITATION**

*According to Newton's third law of motion “to every action, there is an equal
and opposite reaction”. It also holds good for the gravitation. This means that
when earth exerts the force of attraction on an object, then the object also exerts
an equal force on the earth, in the opposite direction.*