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Get the CBSE Class 9 Science notes on chapter 10 ‘Gravitation’ (Part-II) from this article. Here you will get a brief description of the important topics which will help you revise the whole chapter quickly and easily. At the end of the notes you can try the questions asked from the discussed set of topics. These questions will help you to assess your preparation level and get a hold on the subject.

This part is in continuation with **CBSE Class 9 Science, Gravitation: Chapter notes (Part-II), **where we talked about earth’s gravitational force and other related factors.

**CBSE Class 9 Science Syllabus 2017-2018**

**Main topics covered in this part of CBSE Class 9 Science, Gravitation: Chapter Notes, are:**

- Circular Motion & Gravitational Force
- Thrust and Pressure
- Factors affecting Pressure
- Applications of Pressure in daily life
- Buoyancy & Buoyant Force:
- Factors affecting the Buoyant Force
- Archimedes’ Principle
- Applications of Archimedes’ Principle
- Density and Relative Density

**Key notes for Chapter - Gravitation, are:**

**Circular Motion & Gravitational Force**

In circular motion, a force must act on body as its direction of velodity changes. This force is called Centripetal force.

- When moon revolves around earth in circular path, gravitational force of earth provides the centripetal force to moon for circular motion,
- When planets revolve around Sun, gravitational force of Sun provides the centnpetal force to planets.

**Thrust and Pressure**

**Thrust:** Force exerted by an object perpendicular to the surface is called thrust.

**Pressure:** Pressure is defined as thrust or force per unit area on a surface.

- Pressure = Thurst/Area
- SI unit of pressure is Newton/meter
^{2}(N/m^{2}). - SI unit of pressure is called Pascal (Pa).

**Factors affecting Pressure**

Pressure depends on two factors:

(i) Force applied

(ii) Area of surface over which force acts

Since, pressure is indirectly proportional to the surface area of the object, so, pressure increases with decrease in surface area and decreases with increase in surface area.

**Applications of Pressure in daily life**

- The base of high buildings is made wider to spread the weight of the whole building over a large surface area due to which less pressure acts on the ground.
- School bags are provided with broad straps so that the weight of school bags fall over a larger area of the shoulder and produce less pressure hence making it easy to carry.
- The blades of knives are made sharp so that on applying force on it, a large pressure is produced on the very small surface area, thus cutting the object easily.

**Pressure in Fluids**

- Anything that can flow is called Fluid. Example: liquid and gas.
- Molecules of a fluid move randomly and collide with walls of vessel. Thus fluids apply pressure on walls.
- Fluids exert pressure in all directions.

**Buoyancy & Buoyant Force**

- Force applied by the fluid on a solid which is partially or fully submerged in liquid, is called the buoyant force and this phenomenon is named as buoyancy.
- Buoyant force acts in upward direction and it depends on the density of the fluid.

**Factors affecting the Buoyant Force**

Magnitude of the buoyant force depends on two factors:

- Volume of the object immersed in liquid
- Density of the liquid

**Why does an object sink or float over water?**

When an object is immersed in water, it exerts pressure over water due to its weight. At the same time water also exerts upward thrust, i.e., buoyant force over the object.

- If the force exerted by the object is greater than the buoyant force of water, the object sinks in water.
- If the force exerted by the object is less than the buoyant force of water, the object floats over water.

**Archimedes’ Principle**

- It states that when a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

**Applications of Archimedes’ Principle:**

- It is used in designing ships and submarines.
- It is used in determining relative density of substances.
- Hydrometers used to determine the density of liquids, work on this principle.
- Lactometers used to determine purity of milk, are also based on this principle.

*It is because of this principle that ship made of iron and steel floats in water whereas a small piece of iron like nail, sinks in it.*** **

**Density ( ρ)**

- The mass per unit volume is called density of an object.
- Density (
*ρ*) = Mass(M)/Volume(V) - SI unit of density = kg/m
^{3}

**Relative density**

- It is the ratio of the density of a substance to the density of water.

- Since relative density is a ratio of similar quantities, it has no unit.

**Applications of density**

- If an object has density more than that of the liquid, it will float over that liquid.
- If an object has density lower than that of a liquid, it will sink in that liquid.
- When the relative density of a substance is less than 1, it will float in water otherwise it will sink in water.

**Try the following questions:**

**Q1.** What is buoyancy and buoyant force? Upon what factors do they depend?

**Q2.** Why does iron sink in water but floats on mercury?

**Q3.** Why does a buffalo float on the river but not the man?

**Q4.** Why does a mug full of water appear lighter inside the water?

**Q5.** The density of ice is 918kgm^{−3} and that of sea water is 1,030kgm^{−3}. An iceberg floats with a portion 224 liters outside water. Find the volume of iceberg.

**CBSE Class 9 Science Solved Practice Paper 2017-2018: Set-I**

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