Life Sciences Glossary: Human Body Organisation and Mechanics

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Human Body Organisation

Human Body Organisation


Mechanics is the scientific study of motion under the action of force. The motion of a body depends upon its mass and the force acting on it. Mass is the quantity of matter contained in a body. The mass of a body is the same everywhere in the universe. Force is defined as an external agency capable of altering the states of rest or motion in a body. Force is of many types, but we will discuss here gravitational force, friction, centripetal force and ce3ntrifugal force.

Gravitational Force

The force with which the earth pulls an object towards it, is called the gravitational force. The earth attracts all objects towards it with the same amount of force. The force of attraction between the earth and a body on or near the surface of the earth is the force with which a body will fall freely on Earth allowed to do so. From Newton՚s law of motion, the force F acting on a body of mass m falling freely on earth is given by F = mg where g is the acceleration due to gravity of the earth. The force F exerted by the earth on a body of mass m is called the weight of the body and is denoted by W i.e.. W = mg. whereas the mass of a body is constant, the weight of a body is a variable. For example, the weight of a body is maximum at the poles as being flattened the poles are nearer to the centre of the earth and the weight is minimum at the equator as the equator is bulged and away from the centre of the earth. Likewise, on the moon, the moon being less dense and smaller than the earth, a will weigh less than that on the earth.


Friction is the force opposing the relative motion between two surfaces which are in contact with each other. This force exists even when there is no relative motion. Furcation is a surface phenomenon. It acts in a direction opposite to the applied force. The magnitude of the force of friction depends on the roughness of the surfaces in contact. The force of friction is both, a source of use and waste. Without friction, we cannot walk nor can a car move along a road. It is the friction which provides the ‘grip’ with which movement is possible. On the other hand, in machines friction wastes energy and causes wear and tear.

Type of Friction

Limiting Friction

When an applied force is just equivalent to the force of friction that exists between two more bodies, it is known as limiting friction. At limiting friction, the applied forces is about to move the body.

Sliding Friction

The force of friction that acts when a body slides at a steady speed over a surface, is called sliding friction. It is less than the limiting friction.

Rolling Friction

  • The force of friction that opposes the motion of a rolling body over a surface is called rolling friction. It is much less than the sliding friction. Comparatively less energy is spent in rolling an object than sliding or pulling it over a surface3. For this reason, only, vehicles are provided with wheels. Ball bearing also functions on the principle of rolling friction.
  • Friction between bodies generates heat. In order to reduce heat and friction, lubricants such as oil are used between the surfaces of bodies. The oil forms a thin layer between the surfaces of bodies which provides a slippery coating and also reduces direct contact between the surfaces. The study of friction and lubrication is called Tribology.

Centripetal Force

In order to move a body in a circular path, a force must be acting on it towards the centre of the system. This force which acts towards the centre is called centripetal force. The gravitational force of the Sun acts as the centripetal force that makes the planets to go around the Sun, more or less in a circular path. If the Sun՚s gravitational force vanishes, then the planets will no longer move in a circular path but will start moving in a straight line. So, for a body to move in a circular path centripetal force is necessary and must be present in that system.

Centrifugal Force

It is the outward force that acts on a body rotating in circle round a central point. The Centrifugal and centripetal force are equal and opposite.


Inertia is the tendency of a body to resist any change in its state of rest or uniform motion in a straight line. A body possesses a large inertia if it has greater mass. Thus, mass is a measure of inertia of a body.

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