8th Class

  Friction   Synopsis  

• Friction opposes the relative motion between two surfaces in contact. It acts on both the surfaces.

 

• Friction depends on the following factors:

- Material of the bodies in contact. - Roughness of the two surfaces in contact. The rougher the surfaces, the greater is the friction.  

• The force of friction is independent of the area of the two surfaces in contact.

 

• The force of friction between two bodies is parallel to the contact surface and always opposite in direction to that of the relative motion.

 

• Although friction is undesirable, it is important for activities like sitting, walking, braking, more...

  Sound   Synopsis  

• Sound is a form of energy that produces sensation of hearing in our ears.

 

• Sound is produced when a body vibrates.

 

• Sound needs a medium to travel. It cannot travel in vacuum.

 

• Sound travels faster in solid medium (like wood) as compared to water and air.

 

• All musical instruments have vibrating parts/which produce sound.

 

• In human beings, sound is produced by the voice box or the larynx.

 

• We hear through the vibration of our eardrum, whenever sound reaches our ears.

 

• The number of oscillations per second is called the frequency of oscillation (f) and is more...

  Chemical Effects of Electric Current   Synopsis  

• Materials which allow electric current to pass through them are good conductors of electricity, e.g., Metals.

 

• Materials which do not allow electric current to pass through them are bad conductors of electricity, e.g., Wood, Plastic, etc.

 

• Human body is a good conductor of electricity. Thus, we should take precautions while handling electrical appliances.

 

• A bulb glows due to the passage of electric current since the filament of the bulb is heated to a very high temperature. However, if the current through a circuit is too weak/the filament does not get heated sufficiently and it does not glow.

 

• An LED (Light more...

  Some Natural Phenomena   Synopsis  

• High speed winds are accompanied by reduced air pressure.

 

• Air moves from a region of high pressure to low pressure.

 

• A thunderstorm occurs due to strong, upward rising winds along with swift movement of the falling water droplets carried with lightning and sound.

 

• A thunderstorm may develop into a cyclone or a hurricane with wind speed being as high as \[\text{200}\,\text{km}\,{{\text{h}}^{-1}}.\]

 

• A tornado is a dark, funnel shaped cloud that reaches the ground from the sky. A tornado may form within cyclones and can reach a speed up to \[\text{300}\,\text{km}\,{{\text{h}}^{-1}}.\]

 

• Electroscope is an instrument used to detect the presence more...

  Light   Synopsis  

• Light is a form of energy. It is an electromagnetic radiation which can travel through vacuum with a speed of \[3\times {{10}^{8}}\,\,m{{s}^{-1}}\]. Light travels along straight lines. This property is known as the rectilinear propagation of light. We can see an object only when it reflects or scatters the light falling on it.

 

• Any polished or shining surface acts as a mirror and regular reflection takes place.

 

• When there is no regular reflection, sharp images are not obtained and so they cannot be seen by us. This happens because of scattering of light due to irregular reflection.

 

• An image more...

Stars and the Solar System   Synopsis  

• Universe is a vast collection of stars, planets, nebulae, galaxies, intergalactic matter and everything that exists in them. It is believed to have been formed 15 billion years ago from a huge explosion called 'Big Bang’

 

• To measure distances on astronomical scale, the unit light year is used. It is the distance travelled by light in one year. 1 light year \[=9.46\times {{10}^{12}}\,km.\]

 

• Stars are self-luminous, astronomical bodies which produce energy from nuclear fusion reactions. They form from a large collection of gases called nebulae.

 

• A galaxy is a group of billion stars and other celestial bodies. Galaxies exist in different shapes such as more...

Synthetic Fibres and Plastics   Synopsis  

• Natural fibres like cotton are obtained from plants, while wool and silk fibres are obtained from animals.

 

• The rearing of silk worms for obtaining silk is called sericulture.

 

• Silk fibres are made up of a protein.

 

• Synthetic fibres are made by human beings.

 

• A synthetic fibre is also a chain of small units joined together. Each small unit is called a monomer and actually a chemical substance. Many such small units combine to form a large single unit called a polymer.

 

• Synthetic fibres are durable, less expensive and dry up fast.

 

• Rayon (artificial silk) is made more...

Materials: Metals and Non-metals   Synopsis  

• Metals

 

• Metals occur in combined state or in the form of compounds.

 

• Most of the metals are solids at room temperature except mercury which is a liquid.

 

• Most of the metals have high densities because their atoms form close clusters.

 

• Most of the metals shine and can be easily polished.

 

• Metals are malleable, so they can be hammered into thin sheets. They are also ductile and hence, can be drawn into thin wires.

 

• Metals are sonorous because they produce sound when struck.

 

• Most of the metals are good conductors of heat and more...

Coal and Petroleum   Synopsis  

• Sources of energy:

All sources of energy can be divided into two categories: (i) renewable sources of energy and (ii) non - renewable sources of energy.  

• Renewable sources of energy: The sources of energy which are found in nature and inexhaustible are called renewable sources of energy. Some renewable sources of energy are solar energy, geothermal energy, wind energy, tidal energy, bio-energy.

 

• Non-renewable sources of energy: The sources of energy which have accumulated in nature over a long period of time and cannot be quickly replaced once exhausted are called non-renewable sources of energy. Some non-renewable sources of energy are fossil fuels like more...

Combustion and Flame   Synopsis  

• Fuels

Substances which are burnt to produce heat energy are known as fuels. Fuels can be in the form of solid, liquid or gas. Gaseous fuels are better than liquid fuels due to the highest calorific value. Liquids in turn have more calorific value than solid fuels. The main gaseous fuels are natural gas, producer gas, water gas, LP.G and biogas. L.P.G is a mixture of two hydrocarbons - butane and isobutane. The advantages of L.P.G are (i) high calorific value. (ii) burns with a smokeless flame. (iii) does not produce any poisonous gases on burning.  

• Calorific value of fuel

The amount of heat produced by burning 1 g of fuel more...