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• Force: Push or pull on an object which provides motion to an object.
• Balanced forces: Forces of equal magnitudes applied in opposite direction
• Unbalanced forces: Forces of different magnitude causing change in motion of an object.
• Comparison between Balanced and Unbalanced forces:
  

  Balanced Forces	Unbalanced Forces
  Forces of equal in magnitude and opposite in direction	Forces of different magnitude
  Object is at rest	Object moves towards the greater force
  Net force is zero	Net force is non zero
  E.g. Two people pushing object with same force in opposite direction	E.g. Playing rope pulling game

• Frictional forces: Opposite force exerted by the surface, which opposes the motion of an object.
• Using Galileo experiments Newton had present three laws of motion.
• First law of motion [Law of Inertia] : Body maintains its state of rest or uniform motion, until and unless an unbalanced force acts on it.

  Click here to watch video on Newton’s First Law of Motion

• Inertia: Property of body to maintain its state of rest.
• Relation between mass and inertia: Larger the mass higher will be the inertia.
• Examples of First Law of Motion:
  • While taking a sharp turn, body gets thrown to opposite side. This is because body tries to maintains its motion in straight line.
  • When bus starts, body tends to fall backward. This is because feet are in contact with the bus. As motion of bus starts, feet experience motion. But rest of the body tries to be at rest, hence, the body tries to fall backward.
• Momentum (p):
  • Product of mass and velocity
  • It is a scalar quantity.
  • Mathematical formula: p=mv
• Second Law of Motion: Rate of change of momentum of an object is directly proportional to force applied on the object and it occurs in direction of force acting on object.
  • Force = Change in momentum / Time= m × (v-u) / t

    = m × a [ ∵ (v-u) / t ]

  • SI unit: Newton (N) or kg ms^-2

  Click here to watch video on Newton’s Second Law of Motion

• Third Law of Motion: Every action has equal and opposite reaction.
• Examples of third law of motion:
  • Gun recoils back when bullet is fired
  • Motion of rocket
  • Flying of birds

  Click here to watch video on Newton’s Second Law of Motion

• Law of conservation of momentum:
  • Sum of momentum of objects before collision is equal to sum of momentum of objects after collision, in absence of unbalanced forces.
  • m_Au_A+ m_Bu_B = m_Av_A + m_Bv_B
    Where, m_Au_A, m_Bu_B = initial momentum of object A and B respectively.
    m_Av_A, m_Bv_B = final momentum of object A and B respectively.