Definition of speed,velocity, gravity, mass, weight, force, impact, work, power and energy


An object is said to be in motion if it changes its position with time, with respect to its surroundings.Speed is defined as the distance moved per unit time,

i.e Speed =Distance /Time

When we say that an object is at rest or in motion,then this statement is incomplete and meaningless. Basically, rest & motion are relative terms.
An object which is at rest can also be in motion simultaneously. This can be illustrated as follows.
The passengers sitting in a moving bus are at rest with respect to each other but they are also in motion at the same time with respect to the objects like trees, buildings on the road side. So the motion and rest are relative terms.


Velocity indicates the rate of change of the object’s position (r  ); i.e., velocity tells you how fast the object’s position is changing. The magnitude of the velocity (|| v ||  ) indicates the object’s speed. The direction of the velocity (dir v  ) indicates the object’s direction of motion. The velocity at any point is always tangent to the object’s path at that point. Thus, the velocity tells you how the object is moving. In particular, the velocity tells you which way and how fast the object is moving.


Gravitation is the force by which every mass or particle of matter, including photons, attracts and is attracted by every other mass or particle of matter.The gravitational field at any point P in space is defined as the gravitational force felt by a tiny unit mass placed at P.

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.
Mathematically, this law, and the magnitude of the force due to the gravitational interaction between two particles, is expressed with :-

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Properties of Gravitational Force.
It is always attractive in nature while electric and magnetic force can be attractive or repulsive.
It is independent of the medium between the particles while electric and magnetic force depend on the nature of the medium between the particles.
It holds good over a wide range of distances. It is found true for interplanetary to inter atomic distances.
It is a central force i.e. acts along the line joining the centres of two interacting bodies.
It is a two body interaction i.e. gravitational force between two particles is independent of the presence or absence of other particles.
It is the weakest force in nature : As Fnuclear > F electromagnetic > F gravitational .
It is a conservative force i.e. work done by it is path independent or work done in moving a particle round a closed path under the action of gravitational force is zero.
It is an action reaction pair i.e. the force with which one body (say earth) attracts the second body (say moon) is equal to the force with which moon attracts the earth.


It is the measure of the quantity of matter in an object and its unit is kilogram (kg) in SI units. It depends on the number of molecules in the matter and their masses and does not depend on gravity. Therefore an object would have same mass on Earth and on the Moon but different weight because of the change of gravity. Gravity on the moon is 1/6th of the gravity on earth. Mass of an object can be measured by using spring balance (newton-meter), top pan balance or lever balance. The international definition of mass is It is equal to the mass of the international prototype of the kilogram made up of platinum-iridium alloy kept at international bureau of weights and measurements in Paris.

Mass is the amount of matter in an object. It can also be defined as the property of a body that causes it to have weight in a gravitational field. It is important to understand that the mass of an object is not dependent on gravity. Bodies with greater mass are accelerated less by the same force.


The weight of an object is defined as the force of gravity on the object and may be calculated as the mass times the acceleration of gravity, w = mg. Since the weight is a force, its SI unit is the newton.

For an object in free fall, so that gravity is the only force acting on it, then the expression for weight follows from Newton’s second law.


Force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate.


Impact is a high force or shock applied over a short time period when two or more bodies collide. Such a force or acceleration usually has a greater effect than a lower force applied over a proportionally longer period.


Work is said to be done when a force applied on the body displaces the body through a certain distance in the direction of force.Mathematically, work is the force-displacement product

W = F x s cos a

or the force-displacement path integral

dW = F · ds

Positive work means that force (or its component) is parallel to displacement. Negative work means that force (or its component)is opposite to displacement i.e. In conservative field work done by the force over a closed loop is zero.


Power is defined as the rate at which work is done. Its unit is watt Power is said to be one watt, when one joule of work is said to be done in one second.

If work is being done by a machine moving at speed v against a constant force, or resistance, F, then since work done is force times distance, work done per second is Fv, which is the same as power.
Power = Fv


Energy is the capacity for doing work. Energy can manifest itself in many forms like mechanical energy, thermal energy, electric energy, chemical
energy, light energy, nuclear energy, etc.
The energy possessed by a body due to its position or due to its motion is called mechanical energy. The mechanical energy of a body consists of potential energy and kinetic energy.
Potential energy is the energy of a body or a system with respect to the position of the body or the arrangement of the particles of the system.For example, gravitational potential energy is associated with the gravitational force acting on object’s mass; elastic potential energy with the elastic force (ultimately electromagnetic force) acting on the elasticity of a deformed object; electrical potential energy with the coulombic force; strong nuclear force or weak nuclear force acting on the electric charge on the object; chemical potential energy, with the chemical potential of a particular atomic or molecular configuration acting on the atomic/molecular structure of the chemical substance that constitutes the object; thermal potential energy with the electromagnetic force in conjunction with the temperature of the object.

Kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity.


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