Momentum

• Newton’s first law:

  • an object shall remain at rest or at constant velocity provided there is no net force acting on it.
  • At GCSE, we were taught W = resultant. It’s not. W> R due to its velocity changing constantly. (due to earth’s rotation). Therefore, there must always be a resultant force acting on it, despite it remaining “ stationary“

  
• Newton‘s second law:

  • (wordy answer) = rate of change of momentum of an object is directly proportional to the resultant acting on it. Also, the change in momentum = same direction as force
  • When written in SI units = Force = change in momentum. / time
  • Derivation in photo = f=ma where mass is constant

  
• Newton’s third law:

  • when body A exerts a force on body B, body B will exert an equal and opposite force on body A OF THE SAME TYPE
  • e.g. A man standing on earth is pushed down, earth will repel with equal and opposite force
• Linear momentum:

  • mass x velocity = momentum
  • kg x ms^-1 = kgms^-1
• Conservation of linear momentum:

  • unless there are external forces applied, the total linear momentum remains constant in collisions
• Impulse = the product of a force acting on an object and the time taken for the force to act

  • Newton’s second law = f = change in momentum / change in time
  • rearrange so change in time x force = change in momentum = impulse
  • the area under a force time graph = impulse
  • If you increase collision time, force has to be less. This is the reasoning behind air bags
• Elastic and inelastic collisions:

  • elastic = when no ke is losses
  • inelastic = some ke is losses
  • perfectly inelastic = when two things coelest.

  In all three of them, the total energy of the system is conserved (providing no foreign forces)