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Laws Of Motion




3. Laws of motion

  • Newton gave three laws of motion that describe the motion of bodies. These laws are known as Newton's Laws of motion.
  • They describe the relationship between the forces acting on a body and its motion due to those forces.
  • The three laws of motion were first compiled by Sir Isaac Newton in his work Principia Mathematica, first published in 1687. Newton used these laws to explain and investigate the motion of many physical objects and systems.
  • We shall now learn about Newton's First law of motion

4. Newton's First Law of motion

  • The first law of motion is stated as:
    An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.
  • All objects resist a change in their state of motion. The tendency of undisturbed objects whether they are at rest or moving with uniform velocity is called inertia. Hence, the first law of motion is also known as the law of inertia.
  • Greater the inertia of the body greater will be the force required to bring the change in the state of rest or uniform motion of the body.
  • Mass is the measure of the inertia of the body so heavier objects have more inertia then lighter objects. For example a ball of 2Kg has more inertia then a football and it takes more effort to kick a 2Kg ball then it takes to kick a foot ball.
  • Another example is even a small child can push a toy car. However, An adult also can't push a loaded vehicle forward.
  • Newton's first law of motion gives us a definition of force. It says that
    Force is something that changes or tends to change the state of rest or uniform motion of a body

5. Momentum

  • Before discussing about second law of motion we shall first learn about momentum of a moving object.
  • From our daily life experiences like during the game of table tennis if the ball hits a player it does not hurt him. On the other hand, when a fast moving cricket ball hits a spectator, it may hurt him.
  • This suggests that impact produced by moving objects depends on both their mass and velocity.
  • So, there appears to exist some quantity of importance that combines the object's mass and its velocity called momentum and was introduced by Newton.
  • Momentum can be defined as "mass in motion". All objects have mass; so if an object is moving, then it has momentum - it has its mass in motion.
  • The momentum, p of an object is defined as the product of its mass, $m$ and velocity, $v$. That is, Momentum p=mv           (1)
  • Momentum has both direction and magnitude so it is a vector quantity. Its direction is the same as that of velocity, $v$.
  • The SI unit of momentum is kilogram-meter per second (kg m s-1).
  • Since the application of an unbalanced force brings a change in the velocity of the object, it is therefore clear that a force also produces a change of momentum.
  • We define the momentum at the start of the time interval is the initial momentum and at the end of the time interval is the final momentum.
  • When the object moves then it gains momentum as the velocity increases. Hence greater the velocity greater is the momentum.


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