(13)Newton's Laws of Motion

• States of Motion


  • At rest
  • Travelling at constant speed in same direction
  • Travelling in one direction with uniformly increasing speed (constant or uniform acceleration)
  • Travelling in one direction with uniformly decreasing speed (constant or uniform deceleration)
• Kinematics


  The study of motions without looking at the cause of the motion
• Dynamics


  The study of the cause of motions
• Newton formulated three laws to describe the dynamics of the motion of any object
• Newton's First Law


  A body will maintain its state of rest or uniform motion (same speed and same direction) unless acted upon by an external unbalanced force
• Examples of Newton's First Law

  • Why do we need to wear seat belts in cars?
  • Why are we thrown to one side when a car goes around a bend?
  • Why does a soccer ball continue moving when it leaves the foot of the kicker?
  • Why does a book sitting on a table remains at rest
• Newton's Second Law


  The acceleration of a body is directly proportional to the external unbalanced force acting on it provided the mass on the body remains constant
• Newton's Second Law


  1. Accelerate in the direction of the external unbalanced force

  2. The magnitude of acceleration will be proportional to the external unbalanced force if mass on the body remains the same

  3. For a given external unbalanced force the acceleration will decrease if the same proportion as the mass increases and vice versa
• Force

  Unit is Newton (N)
• Different kinds of Forces

  • Friction Force
  • Drag
  • Gravity Force (Weight)
  • Support Force (Normal Force)
  • Thrust
  • Reaction force
  • Lift
  • Tension
• Friction force always acts in the opposite direction to motion
• Drag is a friction force acting on bodies when they travel through a fluid (gas/liquid)
• Gravity force acts towards the centre of the earth (downwards)
• Support force (normal force) always acts at 90 degrees to the surface
• Thrust force is generated by engines
• Reaction force is generated to oppose the applied force
• Lift is support provided by gas (air)
• Tension force is the lengthwise pull in strings, ropes etc