Force is a push or pulls which can change the position of a body as required.


  1. Balanced forces
  2. Unbalanced forces


  1. Nuclear forces
  2. Gravitational force
  3. Electromagnetic force
  4. Weak forces
  • If an object does not change its position with respect to time, it is said to be stationary.
  • If the object changes its position with respect to time than it is said to be in motion.
  • An object may appear to be stationary for one observer and appear to be moving for another.
  • An object is at rest in relation to a certain set of objects and moving in relation to another set of objects.
  • This implies that rest and motion are relative terms.
  • Motion is defined as the change of position of an object with respect to time.



  • A body is said to be in the state of motion, when it continuously changes its position with respect to time.

Measuring the rate of motion

  • The distance covered when traveled along a straight line. Is known as displacement.


  • The shortest distance, or distance traveled along a straight line, is known as displacement.

Uniform Motion and Non Uniform Motion

  • If an object covers equal distances in equal intervals of time, it is said to be in uniform motion.
  • The hare, in its motion, covers different distances in a particular time. This type of motion is known as non-uniform motion.
  • If an object covers unequal distance in equal intervals of time, it is said to be in non-uniform motion.

Linear motion

  • When an object moves along a straight line, it is said to be in linear Motion.
  • Motion of a body dropped from the top of a building and motion of a lift are examples for linear motion.

Circular motion

  • If an object moves along a circular path, it is said to be in circular motion.
  • Motion of a merry go round of hands of a watch and motion of grinder are examples for circular motion.

Centripetal Force and Centrifugal force

  • The constant force that acts on the body along the radius towards the centre and perpendicular to the velocity of the body is known as centripetal force.    
  • Again, centripetal force, F=mr 2 (Since v=r


  • Stone tied to the end of a string and rotated in a circular path, the centripetal force is provided by the tension in the string.
  • When a car takes a turn on the road, the frictional force between the types and the road provides the centripetal force.
  • In the case of planets revolving round the sun or the moon revolves around the earth, the centripetal force is provided by the gravitational force of attraction between them.
  • For an electron revolving around the nucleus in a circular path.
  • The force, which is equal in magnitude but opposite in direction to the centripetal force in known as centrifugal force.
  • While churning curd, butter goes at the side due to centrifugal force.
  • A cyclist turning a cornet leans inwards. Now the frictional force is balanced by the centrifugal force


  • Force is one which changes or tends to change the state of rest or of uniform motion of a body.
  • Force is a vector quantity. It SI unit is Newton.


  • An object remains in the state of rest or of uniform motion in a straight line unless compelled to change that state by an applied unbalanced force.
  • The tendency of undisturbed objects to stay at rest or to keep moving with the same velocity is called inertia.
  • This is why, the first law of motion is also known as the law of inertia.


  • Inertia and mass of a body to change its state of rest or of uniform motion by itself is called inertia.
  • Inertia of body depends mainly upon its mass


  • The momentum ‘p’ of an object is defines as the product of its mass ‘m’ and velocity ‘v’. That is, p = mv
  • Momentum has both direction and magnitude. It is a vector quantity. Its direction is same as that of the velocity.
  • The SI unit of momentum is kg ms-1


  • The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.
  • Therefore F=Ma(4)
  • One unit of force (1N) is defined as the amount of force that produces an acceleration of 1ms-2 in an object of 1 kb mass.
  • The second law of motion gives us a method to measure the force acting on an object as a product of its mass and acceleration.


  • Newton’s third law of motion states that for every action there is an equal and opposite reaction.
  • It must be remembered that the action and reaction always act on two different objects.
  • When a gun is fired it exerts forward force on the bullet. The bullet exerts an equal and opposite reaction force on the gun.
  • This results in the recoil of the gun


  • The law of conservation of momentum states that, in the absence of external unbalanced force the total momentum of a system of objects unchanged or conserved by collision.
  • The total momentum before collision is equal to the total momentum after collision.


  • The magnitude of the moment of force F about a point is defined as the product of the magnitude of force and the perpendicular distance of the point from the line of a action of force.
  • Then, the moment of the force F about the point O = Magnitude of the force X perpendicular distance between the direction of the force and the point about which moment is to be determined = F x d.
  • The unit of moment of the force is Nm.


  • Two equal and opposite forces whose lines of action do not coincide are said to constitute a couple in mechanics.


  • There exist a force between sun and the planets.
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