The Earth takes 365 ¼ days to complete one revolution around the Sun. To keep the equation simple, we take 365 days in a year. The rest one fourth of the day or six hours are added up to one day or 24 hours over a span of four years to the month of February. Thus the month of February has 29 days in a leap year which comes once in four year.

Leap year is basically a year with an extra day in it, February 29, and occurs once every four years. The reason for this is to keep the seasons, astronomical events and time differences in sync. This has to do with the Gregorian calendar, which is used almost everywhere in the world. Leap Year is directly related to the Earth and how many times it orbits the sun in one year. There are not actually 365 days in a year, but 365 1/4 days in a year. So basically, four quarters would make up an extra day, hence adding a day every four years.

A physical quantity which have both magnitude and direction is the vector quantity.

An element is a substance that is made entirely from one type of atom. For example, the element hydrogen is made from atoms containing a single proton and a single electron. If you change the number of protons an atom has, you change the type of element it is.
Currently, scientists know of 118 different elements. Some, like gold, silver, copper and carbon, have been known for thousands of years. Others, such as meitnerium, darmstadtium and ununquadium, have only recently been created by scientists. All known elements are arranged on a chart called the Periodic Table of Elements.

There are three types of inertia:
1. inertia of rest: this is the resistance offered by the body to remain in rest unless an external force is not applied on the body. eg.:if a carpet is beaten with a stick then the dust particles due to inertia of rest tends to remain in rest while the fibers of carpet becomes in motion.
2. inertia of direction: this is the resistance offered by the body to change its direction unless an ext.force is not applied on the body.Eg:if a bus tkes a turn then passengers sitting in tends to remain in the same direction in which the bus was travelling before.
3. inertia of motion: the resistance offerd by the body to change its uniform motion.Eg:if a bus stops suddenly then the passengers fall forward due to inertia of motion.

The important characteristics of particles of matter are the following:

1. The particles of matter are very, very small.
2. The particles of matter have space between them.
3. The particles of matter are constantly moving.
4. The particles of matter attract each other.

After a few minutes of spraying a perfume on yourself, you will not smell it. The nose registers that smell and in a while gets saturated with the same smell. Hence in perfume shops they offer to smell coffee beans, which breaks the smell so your nose can smell other odours. When you spray the perfume, tiny droplets of the liquid are atomized into the air where they quickly evaporate. The gas particles move randomly about, diffusing into surrounding air in the room, becoming more dilute as they spread out. Soon the entire room contains some of the perfume molecules.

• Inertia is the natural tendency of an object to resist a change in its state of motion or of rest.
• The mass of an object is a measure of its inertia.
• A force that is just enough to cause a small cart to pick up a large velocity will produce a negligible change in the motion of a train, because of train’s large inertia.

An object is said to be in motion if it changes its position with respect time and observer.
An object is said to be at rest if it does not changes its position with respect time and observer.
MOTION AND REST ARE RELATIVE TERMS.
That means it all depends upon the observer who is observing the object.
Example:
1. A person standing in a moving bus is at rest with respect to bus and is in motion with respect to observer on the ground.
2. A girl swinging in garden is in motion with respect to persons moving in garden and at rest with respect to swing.

It is easier to sip hot tea or milk faster from a saucer rather than from a cup because of evaporation . As we know that evaporation depends on surface area. If the surface area is more rate of evaporation is faster . And evaporation causes cooling. So if the surface area is more rate of evaporation is faster and so is the cooling rate. Hence we are able to sip hot tea or milk faster from a saucer rather than from a cup.

The boiling point is the temperature at which a material changes from a liquid to a gas (boils) while the melting point is the temperature at which a material changes from a solid to a liquid (melts). A material's melting point is the same as its freezing point.

Reverse osmosis is the process in which pressure is applied to overcome colligative property and osmotic pressure that is directed by thermodynamic parameter and a chemical difference of a solvent.

This application is mainly applied in the production of potable water in water plants and in industries. The end result will be the solute. It happens when the pure solvent is allowed to follow to one end of the membrane thus allowing a solute to retain in a permissible side of a membrane. Reverse osmosis removes suspended and types of dissolved species from water including bacteria.

A mole is defined as the amount of a substance that contains exactly 6.02214076 * 10²³ ‘elementary entities’ of the given substance.

Radiation does not require a medium in which the energy needs to transmit through. Solar radiation warming the Earth's surface is an example. Radiation is heat radiated in the form of rays or waves. This energy transfer occurs via waves that do not require a medium through which to travel. All physical substances in solid, liquid, or gaseous states can emit energy via a process of electromagnetic radiation because of vibrational and rotational movement of their molecules and atoms. The radiation occurs at all temperatures, with the rate of emission increasing with the temperature.

As the humidity increases, so too does the percentage of air molecules that are water molecules. ... Lower density translates into faster sound wave travel, so sound waves travel faster at high humidity. The increase in speed, however, is very small, so for most everyday purposes you can ignore it.The speed of sound is affected by temperature and humidity. Because it is less dense, sound passes through hot air faster than it passes through cold air. ... Dry air absorbs far more acoustical energy than does moist air. This is because moist air is less dense than dry air (water vapor weighs less than air).

The speed of sound also depends on the temperature of the medium. The hotter the medium is, the faster its particles move and therefore the quicker the sound will travel through the medium. When we heat a substance, the particles in that substance have more kinetic energy and vibrate or move faster. Sound can therefore be transmitted more easily and quickly in hotter substances.

According to the second law of motion, force is the product of mass and acceleration. Suppose the mass of the stone be m. The acceleration involved in falling objects due to the gravitational force and is denoted by g.

Therefore the magnitude of the gravitational force F will be equal to the product of mass and acceleration due to the gravitational force, that is,

F = m g                                                                                                                                 ii.

From Eqs. i. and ii.  we have

m g = G M x m/d²

Or g = GM /d²

where M is the mass of the earth, and d is the distance between the object and the earth.

Let an object be on or near the surface of the earth. The distance d will be equal to R, the radius of the earth. Thus, for objects on or near the surface of the earth,

mg = = G M x m/r²

g = GM /r²

To Calculate the Value of g:

To calculate the value of g, we should put the value of universal gravitational constant, G = 6.673 × 10^–11 N m² kg^–2; mass of the earth, M = 6 x 10²⁴ kg; and radius of earth R = 6.4 x 10⁶ m in the given equation:

g = GM /r²

g = 6.673 × 10^–11 N m² kg^–2 x 6 x 10²⁴ kg/(6.4 x 10⁶ m)² = 9.8 m/s²

Thus, the value of acceleration due to gravity of the earth, g = 9.8 m s^–2.

Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them.

Suppose two bodies A and B of masses M and m are lying at a distance d from each other. Let the force of attraction between these two bodies be F. according to the universal law of gravitation;

1. The force between the two bodies is directly proportional to the product of their masses.

F ∝ M × m

1. The force between the two bodies is inversely proportional to the square of the distance between them.

F ∝ 1/d²

Combining both the equations:

F ∝ M x m/d²

Or F = G M x m/d²                                                                                                           i.

where G is the constant of proportionality and is called the universal gravitation constant.

By multiplying crosswise we get

 F × d² = G M × m

Or G = F d²/ M x m

The SI unit of G can be obtained by substituting the units of force, distance and mass as N m² kg ^–2.

The value of G was found out by Henry Cavendish (1731 – 1810). The accepted value of G is 6.673 × 10^–11 N m² kg^–2.