Plant leaves appear green. Why?

Sunlight contains seven colours, viz violet, indigo ,blue, green, yellow, orange and red. The leaves appear green because they absorb all the colours and reflect green as green colour is least used in photosynthesis. This green colour reaches our eyes. Hence leaves appear green.

Shallow water runs faster than deep river water. Why?

The equation of continuity is
area of cross-section of flow of water × velocity of water = constant.
Hence  the velocity of water is inversely proportional to the the area of cross-section of water flowing.
When water runs deep then the area of cross section of the flowing water is more than when its shallow so the velocity is less for deep water.

Does specific resistance/resistvity change on stretching a wire?

Specific resistance or resistivity is the resistance of a wire of unit length and unit area of cross-section.

It depends on two factors.
1. Nature of the substance.
2.  Temperature of the substance

It is not effected by
1. Length of the wire.
2. Area of cross-section of the wire.

So., when you stretch a  wire its RESISTIVITY remains constant.

Camels have broad feet. Why?

theloveforphysics

Camels have broad feet so that they can walk on sand. As the area of their feet is more, the pressure due to their weight on the sand is less. So they can walk easily without sinking their feet on the sand.

Freezer compartment of a refrigerator is placed at its top portion. Why?

Cold air from the freezing compartment settles down owing to its higher density and keeps the temperature of the contents below it low.

Infrared rays are used for thermal photography?

Infrared rays are invisible rays whose wavelength is more than that of red colour.They produce heating effect. As they have long wavelength, they are least scattered by the fog, mist and smog and dust particles. As they effect a specially treated photographic plate due to the heating effect, they are used in thermal photography.

THE DIFFERENCE BETWEEN AC AND DC.

                        ALTERNATING CURRENT
1. Its value changes continuously.
2. Its polarity changes periodically.
3. It can be transformed by a transformer.

                         DIRECT CURRENT

1. Its value doesn't change, it has  a constant magnitude.
2. Its polarity doesn't  change.
3. It can not be transformed by transformer.

The temperature is lower in shade than in direct sunlight, why ?

In the direct sunlight there are Infrared Rays. Infrared rays have strong heating effect. But in shade there is no direct sunlight with these heating Rays. Only reflected or diffused light is there. So, temperature is lower in  shade than in direct  sunlight

A spanner has long handle. Why?

A spanner is used to turn a nut to open or tighten it.
The rotational effect depends on the perpendicular distance between the force and the point of application of force. When we increase the perpendicular distance then the Moment of force increases keeping the moment same.

Efficiency of a machine is always less than 100%. Why?

Efficiency is the ratio of work output and work input. As work output is always less than work output for the following reasons, the efficiency is less than 1 or 100%.

1. The movable parts of a machines are not weightless.
2.  The movable parts are not frictionless.
3.  The parts of the machines are not rigid.
4. The strings or belts are not perfectly elastic in nature.

Why is electromagnet made of soft iron?

Electromagnet is a temporary magnet made of soft iron. Soft iron is magnetised by the magnetic field of a coil or solenoid.
Soft iron has high magnetic permeability due to which it gets magnetised quickly and it loses magnetic properties easily due to the fact that it has low magnetic retentivity .

CLOUDS APPEAR WHITE, WHY?

Cloud appears white due to the phenomena of scattering of white light. The water particles in the cloud are big in size as compared to air molecules and wavelength of light. Hence all the colours get scattered in equal intensity and mixture of all colours gives white light. This white light reaches our eyes giving the impression that colour of a cloud is white.

Temperature of surrounding falls after a heavy shower. Why?

Water has high specific heat capacity . It absorbs large amount of heat from the surrounding without  much rise in its temperature .Hence the temperature falls .

Our moon does not have atmosphere. Why?

The escape velocity of the moon is less than the average velocity of the atom or molecule of  gases. Hence all gas molecules escape out of moon's gravitational field .

WHY IS EARTHING OF AN APPLIANCE DONE?

Earthing is required in a circuit when
1. the appliance draws high current. High power devices draw large current.(Current =power/voltage).
2. the appliance has outer metallic case.

Temperature falls as altitude increases, Why?

The volume of rising warm air increases with height due the availability of more space above. This expansion (Joule-Thompson effect) results in cooling effect. Hence hillstations have low temperature. This phenomena helps in the formation of clouds and various forms of precipitation.

FUSE WIRE

Fuse wire is a safety device to limit the current in an electric circuit. It gets melted when excess current flows in a circuit. It works with the principle of heating effect of electric current. It is made with an alloy of lead and tin (solder wire).
Its material has two important properties
1. It has low melting point (around 250 degree celcius).
2.  It has high resistivity.

Sky appears blue, why?

Question: Why does sky appear blue ?
Answer: Sky appears blue due to the scattering of light by air molecules. Blue components (violet, indigo, blue colours) of white light are scattered the most owing to their short wavelengths compared to other colours. This scattered blue light reaches our eyes and the sky appears blue.

NEWTON'S SECOND LAW OF MOTION

The Second law states that the rate of change of momentum is directly proportional to the applied force and the change takes place in the direction of the force applied.

If u= initial velocity,
v=final velocity,
t= time taken to change the velocity from u to v,

Then
Initial momentum = mu
Final  momentum=mv

So change in momentum = mv-mu =m(v-u)
Hence the rate of change of momentum = m(v-u)/t

Therefore F= m(v-u)/t
  F=ma.
                            

NEWTON'S THIRD LAW

The law states that "to every action there is an equal and opposite reaction and these forces act on different bodies"

Examples

1. When a rocket is launched, the burnt gases are expelled down from the rocket body. Here action is on gases by the rocket body. Hence the gases exert an equal an opposite force on the rocket body in the upward direction. This is the reaction force. With this force rocket moves in upward direction.

ACTION...on the gases by the Rocket body.
REACTION..On the rocket by the burnt gases.

2. When a gun fires bullets, action is on the bullets by the gun and reaction on the gun by the bullets. Gun recoils back due to the reaction.

ACTION: On the bullets by the gun..
REACTION...on the gun by the bullets