when a fire brigade arrives, what does it do?

Ans: It pours water on the fire. Water cools the combustible material so that its temperature is brought below its ignition temperature. This prevents the fire from spreading.Water vapors also surround the combustible material, helping in cutting off the supply of air. So, the fire is extinguished.

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Matchstick

 

Ques: Why does the matchstick start burning on rubbing it on the side of the matchbox?

Ans: These days the head of the safety match contains only antimony trisulphide and potassium chlorate. The rubbing surface has powdered glass and a little red phosphorus (which is much less dangerous). When the match is struck against the rubbing surface, some red phosphorus gets converted into white phosphorus. This immediately reacts with potassium chlorate in the matchstick head to produce enough heat to ignite antimony trisulphide and start the combustion.

Ques: Which compounds are used in matchstick in the earlier time?

Ans: A mixture of antimony trisulphide, potassium chlorate and white phosphorus with some glue and starch was applied on the head of a match made of suitable wood. When struck against a rough surface, white phosphorus got ignited due to the heat of friction. This started the combustion of the match. However, white phosphorus proved to be dangerous both for the workers involved in the manufacturing of matches and for the users.

 

Few facts

Ques: Which compound used as matches in ancient Egypt?

Ans: small pieces of pinewood dipped in sulphur were used as matches in ancient Egypt.

Ques: What is the difference between the burning of a candle and the burning of a fuel like coal?

Ans: candle burns with a flame whereas coal does not.

Ques: The sun produces its own heat and light. Is it also some kind of combustion?

Ans: In the sun, heat and light are produced by nuclear reactions.

Force and its types

FORCE: A force is a push or pull acting upon an object as a result of its interaction with another object.  The SI unit of force is Newton (N).

There are a variety of types of forces. A variety of force types were placed into two broad category headings on the basis of whether the force resulted from the contact or non-contact of the two interacting objects.

Contact Forces Non-Contact Forces
Frictional Force Gravitational Force
Tensional Force Electrical Force
Normal Force Magnetic Force
Air Resistance Force
Applied Force
Spring Force

Applied force: An applied force is a force which is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.

Gravity force: The force of gravity is the force at which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity which is directed “downward” towards the center of the earth. The force of gravity on earth is always equal to the weight of the object as found by the equation:

Fgrav = m * g

where g = 9.8 m/s2 (on Earth)

and m = mass (in kg)

Normal force: The normal force is the support force exerted upon an object which is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasions, a normal force is exerted horizontally between two objects which are in contact with each other.

Friction force: The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. The friction force opposes the motion of the object. For example, if a book moves across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion. Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. The friction force can be calculated using the equation:

Air resistance force: The air resistance is a special type of frictional force which acts upon objects as they travel through the air. Like all frictional forces, the force of air resistance always opposes the motion of the object. This force will frequently be neglected due to its negligible magnitude. It is most noticeable for objects which travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.

Tensional force: The tension is the force which is transmitted through a string, rope, or wire when it is pulled tight by forces acting from each end. The tensional force is directed along the wire and pulls equally on the objects on either end of the wire.

Spring force: The spring force is the force exerted by a compressed or stretched spring upon any object which is attached to it. An object which compresses or stretches a spring is always acted upon by a force which restores the object to its rest or equilibrium position. For most springs (specifically, for those which are said to obey “Hooke’s Law”), the magnitude of the force is directly proportional to the amount of stretch or compression.

Plant and Animal Cell

How does a plant cell differ from an animal cell?

The main difference is that plant cells have a cell wall as well as a cell membrane. Animal cells only have a cell membrane.

  1. Plant cells have cell walls, which supports a rigid (rectangular) structure. These structures are composed of cellulose, hemicellulose, and a variety of other materials. An animal cell does not have this cell wall; as such, the shape is more dynamic. With animal cells there is a diversity of shapes although most are roughly circular to maximize surface area.
  2. Plant cells have chlorophyll, the light-absorbing pigment required for photosynthesis. This pigment, which makes plants appear green, is contained in structures called chloroplasts (or more generally, plastids).
  3. Similarly, chlorophyll containing plant cells go through both photosynthesis and cellular respiration, while animal cells only go through cellular respiration.
  4. Plants cells have a large, central vacuole. While animal cells may have one or more small vacuoles, they do not take up the volume that the central vacuole does (up to 90% of the entire cell volume!). The vacuole stores water and ions, and may be used for storage of toxins.
  5. Animal cells have centrioles, cilia (unicelluar animal cell), and lysosomes. Plant cells have no need for centrioles because their spindle fibers connect to the cell wall.