Motion – Core of Forces

Hey readers, today we will go through the terms related with the key topic “Motion”, the core of forces.


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Scalar Quantities – Physical quantities which have magnitude only and have no direction are called scalar quantities. Eg: Mass, speed, volume, work, time, power, energy etc.


Vector Quantities – These have magnitude and direction both, which obey triangle law. Eg: Displacement, velocity, force, momentum, torque etc.

  • Moment of inertia, pressure, refractive index, stress are called tensor quantities.
  • Electric Current has a direction but it is a scalar quantity because it does not obey the law of triangle.


Distance – It is the length of actual path covered by moving object in a given time interval. It is a scalar quantity.


Displacement – It is a shortest distance covered by a body in a definite direction. It is a vector quantity. Displacement may be positive, negative or zero, whereas distance is always positive.


Velocity – The rate of change of distance is called velocity. It is a vector quantity and is SI unit is metre/second. It may be positive or negative.

  • Formula – Displacement/Time


Acceleration – It is the rate of change of velocity. It is a vector quantity.

  • SI Unit – m/s2
  • Formula – Change in Velocity/Time
  • Relation with acceleration – When the velocity of body increases, then its acceleration is positive and vice versa.
  • If the velocity of body decreases, then its acceleration is called retardation.


Centripetal Force – A body performing circular motion, its body changes continuously. The external force required to maintain the circular motion of the body is called centripetal force.

  • This force is always directed towards the centre.
  • FormulaMV2/R where m=mass, s=uniform speed and r=radius


Centrifugal Force – In applying the Newton’s laws of motion, there are some forces which cannot be assigned to any object in the surrounding.

  • These forces are called pseudo forces or inertial force.
  • Centrifugal force is a pseudo force. It is equal and opposite to centripetal force. Example – Cream separator, centrifugal drier.


Newton’s law of motion


First Law

His first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This law is also known as law of inertia or law of Galileo.

  • Intertia – It is a property of body by virtue of which the body opposes change in its initial state of rest or motion with uniform speed on a straight line.
  • It is of three types – Inertia of rest, Inertia of motion and Inertia of Direction.


Second Law

The rate of change in momentum is directly proportional to the applied force on the body and takes place in the direction of force.

  • Force – External energy on a body which changes or tries to change the initial state of the body.
  • Momentum – It is strength or force that something has when it is moving. It is the product of the mass and velocity of the body. It is a vector quantity. Its SI unit is Kgm/s.


Third Law

For every action there is an equal and opposite reaction. Example – While sitting on chair your body exerts a force downward and that chair needs to exert an equal force upward or the chair will collapse.



If the force acts on a body for a very small time, it is known as impulive force.

  • It is a vector quantity.
  • Impulse = Force*Time = Momentum
  • SI Unit – Newton second (Ns)



If resultant of all the forces acting on a body is zero then the body is said to be in equilibrium.

  • If the body is in equilibrium, it will be either at rest or uniform motion.
  • At rest the equilibrium is called static otherwise dynamic.
  • Static equilibrium is of two types – stable and unstable



With this we have completed Today’s topic. Hope you have learned today.

We will post Quiz on Motion in Today’s Cracking Session

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