# Lakhmir Singh Solutions for Class 9th Physics Chapter 3: Gravitation

This chapter focuses on the universal force of gravitation due to which every object in this universe attracts every other object. Gravitational force is directly proportional to the product of masses of the objects and inversely proportional to the distance between them. This is called the Universal law of gravitation. G is the universal gravitational constant, and its SI unit is Nm2 /Kg2. This force holds all the planetary objects together in the universe.

The chapter further includes Kepler's laws of planetary motion by Sir Johannes Kepler and how Newton guessed the inverse square law. When a body goes through free fall due to Earth's gravitational force, an uniform acceleration is produced, which is known as acceleration due to gravity. Its value is g= 9.8 m/s2 . The value of acceleration due to gravity keeps increasing from the equator to the poles and decreases as we move upwards or inside the Earth's surface.

The chapter then explores topics such as equations of motion for freely falling bodies and the concept of mass and weight. The mass of an object is the same everywhere. Whereas the weight on the moon will be about one-sixth of what it is on the Earth. Do students know the concept behind the width of the straps? Why is a sharp knife more efficient in cutting than a blunt knife? All these are due to the pressure, which can be defined as the force acting perpendicularly on a unit area of the object.

Liquids also exert an upward force on the objects immersed in them; this force is called buoyancy or upthrust. Experiments can be performed to study the magnitude of buoyant force and various factors affecting it, like the density of liquid or volume of an object immersed. Another important topic of this chapter is the Archimedes Principle, which states that when an object is immersed fully or partially in a liquid, it experiences a buoyant force equal to the total weight of the liquid displaced by an object. The submarines, lactometers, hydrometers are some devices that work based on Archimedes principle.

The principle of floatation states that an object will float in a liquid if the weight of the object is equal to the weight of the liquid displaced by it. This forms the basis of understanding how a boat floats in water. The density of any object is defined as the mass of the object that acts per unit area. Its SI unit is kg/m2. The relative density of a given object is described as the ratio of the object's density to the density of water. Hence, an object floats in water if its density is less than that of the water. Otherwise, it will sink.