# Facts about Force and Gravity

Force is any influence that changes or tends to change the state of rest or the uniform motion in a straight line of a body.

Newton’s second law gives the measure of force.
F= ma; Here F is force, m is mass and a is acceleration.

Unit of force is newton. dyne is yet another unit of force.

One newton = 105 dynes.

The strongest force in nature is nuclear force.

The weakest force in nature is gravitational force.

The large force acting for a very short time is called an impulsive force. It is calculated as the rate of change in momentum.
eg: force exerted on a bullet when it is fired with a gun force exerted by hammer on a nail etc.

Impulse of a force is the multiplication of force and time during which the force acts on the body.

The force of attraction between like molecules is called cohesion.

The force of attraction among unlike molecules is called adhesion.

Weight can be defined as the force gravitational acting on a body. Weight, w = m x g

Mass remains constant while weight varies from place to place.

Friction is a force that opposes the move of one surface above another.

Kinetic friction between two surfaces in contact is the force of friction which comes into play when there is relative motion between the surfaces.

Rolling friction between two surfaces in contact is the force of friction which comes into play when a body rolls over another. This is the principle on which ball bearings work.

Due to the existence of friction we can walk on the ground, trains can run on rails.

Frictional force offered by air helps the parachute to have a slow landing.

Friction offered by atmospheric air is protecting us from being hit by meteors.

A lubricant is a substance which forms a thin layer between two surfaces in contact.

Generally oil and grease are used as lubricants to reduce friction as well as to protect the moving parts from over heating. A modern lubricant is a mixture of mineral oil, vegetable oil and colloidal thin oil. Flow of compressed pure dry air would acts as lubricant.

Graphite is a solid lubricant.

Centripetal force is a force which acts on a body that moves in a circular path and is going towards the centre about which the body moves. Centripetal force is necessary to keep an object moving in a curved path.

When an individual circles round the earth in a spacevehicle, his mass remains constant other than weight becomes zero.

Centrifugal force is a force that acts on a body moving in a circular path and is directed away from the centre around which the body is moving. Centrifugal force helps to separate cream from milk.

Force of attraction between any two bodies in the universe is known as Gravitation.

The attractive force of earth, or further space body, on an object is called Gravity.

The earth attracts every body towards its centre. The acceleration with which the freely falling bodies are attracted towards the earth is called the acceleration due to gravity (g).

Normally the value of g is taken as 9.8 m/s2. The dimension of `g' at the centre of earth is zero. The value of 'g' is maximum at polar region and minimum at equatorial region. The dimension of `g' decreases due to rotation of earth.

The value of 'g' reduces with altitude or depth from earth's surface.

Newton's Law of Universal Gravitation explains that each object in the cosmos magnetizes further particles with a force that is directly proportional to the multiplication of their masses and it is inversely proportional to the square of distance amid them.

F = G Mm/d2, where `G' is gravitational constant. Value of ‘G’ is 6.67 x 10-11 Nm2/kg2

In moon, an astronaut can't drink lemonade without straw why?

Ans: Because there is no atmosphere on the moon and hence no atmospheric pressure.

The acceleration due to gravity is maximum on Jupiter and minimum on planet Mercury.

As the mass of the substances increases the gravitational force between them also increases.

The weight of a body on earth is maximum in the polar region and minimum in the equatorial region.

In the absence of an effective force of gravity, bodies become weightless in artificial satellites.

The weight of a body on the moon will be less than its weight on the earth. This is accordingly since the mass of the moon is 1/81th of the mass of the earth; its mean radius is 1/3.66th of the radius of the earth. Therefore, the acceleration due to gravity experienced on the moon will be less than the acceleration due to gravity experienced on the earth. This will be about 1/6 of that on earth.

The orbital velocity of a satellite does not depend on the mass of the body. It depends upon the radius of the orbit.

Larger the radius of orbit, smaller will be the orbital velocity.

The orbital velocity of a satellite revolving near the surface of earth is 7.9 kinisec.

Time taken by a satellite to revolve one circle in its orbit is called its period of revolution.

Period of revolution of a satellite or other space object depends upon the altitude of satellite from the surface of earth.

Larger the altitude, more will be the period of revolution.

Period of revolution of a satellite or other space object is independent of its mass.

A satellite with an orbital period same as the Earth's rotation period is called geo-stationary satellite.

Its period of rotation is 24 hours.

Geo-stationary satellites revolve around the earth at a height of 36000 km.

The path of geostationary satellite is known as parking orbit.

Gravitation is important for maintaining the earth and further planets in their orbits surrounding the sun; for maintaining the moon in its orbit surrounding the earth; for the creation of tides; for natural movement by which fluid run take place under the power of a density gradient and gravity; for heating the interiors of emerging stars and planets to extremely high temperatures; and also for a range of other processes occurred on earth.

Gravitational field is the space around a massive body in which gravitational force of attraction is felt.

Planets move slower along their orbits when they are farthest from the sun (at apogee) and they move faster along their orbits when they are nearest to the sun (at perigee).

Escape velocity is the minimum speed which a body must have to escape from the earth's gravitational force.

Escape velocity of moon is 2.4 km/s.

Escape velocity of earth is 11.2 km/s (7miles/sec).

The escape velocity of the planet Jupiter is 60 km/second and of Mercury it is 4.2 km/second.