# NCERT Class 9 Science Solutions: Chapter 10 – Gravitation Part 2

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Question 1:

Q. What are the differences between the mass of an object and its weight?

**Answer:**

Sr.No. | Mass | Weight |

I. | Mass is the quantity of matter contained in the body. | Weight is the force of gravity acting on the body. |

II. | It is the measure of inertia of the body. | It is the measure of gravity. |

III. | Mass is a constant quantity. | Weight is not constant quantity. It is different at different places. |

IV. | It only has magnitude. | It has magnitude as well as direction. |

V. | Its SI unit kilogram (kg). | Its SI unit is the same as the SI unit of force, i.e., newton (N). |

Question 2:

Q. Why is the weight of an object on the moon its weight on the earth?

**Answer:**

Let be the mass of the Earth and be an object on the surface of the Earth. Let be the radius of the Earth. According to the universal law of gravitation, weight of the object on the surface of the Earth is given by,

Let and the mass and radius of the moon. Then, according to the universal law of gravitation, weight of the surface of the moon is given by:

Where,

Therefore, weight of an object on the moon is of its weight on the Earth.

Question 1:

Q. Why is it difficult to hold a school bag having a strap made of a thin and strong string?

**Answer:**

It is difficult to hold a school bag having a thin strap because the pressure on the shoulders is quite large. This is because the pressure is inversely proportional to the surface area on which the force acts. The smaller is the surface area; the larger will be the pressure on the surface. In the case of a thin strap, the contact surface area is very small. Hence, the pressure exerted on the shoulder is very large.