**Very Short Questions:-**

**Q. Define universal gravitational constant.**

Ans:- The force of gravitation produced between two bodies of unit masses separated by unit distance from their center is called universal gravitational constant.

**Q. State Newton’s law of gravitation.**

Ans:- Newton’s law of gravitation states that **“The force of gravitation acting between two bodies is directly proportional to the product of their masses and inversely proportional to square of the distance between their centers.”**

**Q. Define acceleration due to gravity.**

Ans:- Acceleration produced in a body because of force of gravity is called acceleration due to gravity.

**Q. Define freefall.**

Ans:- Body is said to be in freefall when it is accelerated only by force of gravity and its acceleration is equal to acceleration due to gravity.

**Q. Write down the mathematical expression of Newton’s law of gravitation.**

Ans:- The mathematical expression of Newton’s law of gravitation is

F = GMm/d^{2}

where, F = force of gravitation, G = universal gravitational constant, M = mass of one body, m = mass of other body, d = distance between their centers.

**Short Questions:-**

**Q. The value of universal gravitational constant is 6.67 X 10 ^{-11} Nm^{2}/Kg^{2}. What do you mean by it?**

Ans:- The value of universal gravitational constant is 6.67 X 10^{-11} Nm^{2}/Kg^{2 }.It means the force of gravitation acting between two bodies of unit masses separated by unit distance is 6.67 X 10^{-11} N.

**Q. What is the direction of acceleration due to gravity and why?**

Ans:- The direction of acceleration due to gravity is downward or, towards the center of heavenly body because acceleration is produced in the direction of force (By Newton’s second law of motion) applied. So, force of gravity is acting towards the center of heavenly body.

**Q. What is the acceleration of a free falling object on the moon?**

Ans:- Acceleration of a free falling object on the Moon is 1.63 m/s^{2}

**Q. Write geocentric and heliocentric theory.**

Ans:- **Geocentric theory:-** According to this theory Earth is in center and other heavenly bodies like Sun revolve around it.

**Heliocentric Theory:- **According to this theory Sun is in the center of solar system and other heavenly bodies like planets revolve around it.

**Q. A person in an artificial satellite orbiting around the earth feels weightlessness.**

Ans:-