(14)Weight, mass and terminal velocity

• Mass


  The amount of matter contained in a body
• Weight


  The force exerted on bodies due to the gravity of earth or any other body
• Unit of weight


  Newton (N)
• Any object irrespective of its mass when dropped close to the earth's surface has an acceleration of 10 ms-2
• This value of acceleration is constant up to a height of about 20 km
• Acceleration due to gravity


  Represented by the symbol "g"
• On Earth, g = 10 ms-2 downwards (towards the centre of the Earth)
• An object is accelerating


  There is an external unbalanced force acting on the object
• The external unbalanced force


  Is exerted by the earth due to its gravity
• Relationship between acceleration and external unbalanced force

  1. External unbalanced Force = Mass x Acceleration

  2. Gravity Force = Mass x Acceleration due to gravity

  3. Weight = Mass x 10
• Calculate the following on Earth:

  • Weight of a boy whose mass is 65 kg
  • Mass of a boy whose weight is 165 N
• Drag


  Friction force that acts on moving object in fluids (liquids and gases)
• Drag force


  • Size depends on the shape of the body and its speed through the fluid
  • Size is reduced by having streamline shapes eg planes, fish, rockets etc
  • Size increases as the speed increases
• Terminal Velocity


  The constant speed reached when the drag force becomes equal to the gravity force
• Parachutist falling without parachute

  1. Gravity force is the only force acting, giving acceleration of 10 ms-2 downwards

  2. Drag force starts increasing as speed increases, reducing the external unbalanced force

  3. Terminal velocity is reached when drag force equals gravity force

  
• Effect of parachute


  1. Parachute generates large drag force when opened, creating upward acceleration

  2. As speed decreases, drag force decreases but remains equal to gravity force, reaching a new terminal velocity

  
• Acceleration due to gravity