Centre of Gravity, Forces, Hookes Law

• The centre of gravity is the point at which an object can be balanced
• Newtons first law states that if there is no resultant force acting on an object it will remain stationary or continue moving with constant velocity
• Centre of Gravity is the point on an object where all the objects weight appears to act
• Resultant Force = sum of forces acting on an object
• Centre of gravity


  The point on an object where all the object's weight appears to act
• Centre of gravity


  • Acts like a pivot, with equal clockwise and anticlockwise moments acting
  • As the moments are balanced, the object remains balanced in equilibrium
• Finding the centre of gravity of an irregular object

  1. Draw an irregular shape onto a card and cut it out

  2. Using a pencil, make a hole near the edge of the shape

  3. Suspend the shape on a pin and suspend a plumb line from the same pin

  4. Mark the plumb line on the shape

  5. Repeat by creating two further holes at different locations on the shape

  6. The point where the three lines cross is the centre of gravity
• Stable object


  • Has a low centre of gravity and wide base
  • When the centre of gravity acts through the base, the object will remain in position
• Common forces


  • Gravity
  • Weight
  • Magnetic
  • Friction
  • Electrostatic
  • Air resistance
  • Drag
  • Tension
  • Thrust
• Gravity


  Attractive force created by objects with mass
• Weight


  Force of gravity acting on an object
• Magnetic force


  Attractive or repulsive force created by a magnet
• Friction


  Force that opposes motion between objects
• Electrostatic force


  Attractive or repulsive force created by a charged object
• Air resistance


  Frictional force acting on a moving object due to the air around it
• Drag


  Frictional force acting on a moving object due to the fluid around it
• Tension


  Force within an object being stretched
• Thrust


  Upwards force on a floating object created by the liquid it is in
• Normal reaction


  Upwards force on an object due to the surface it sits on
• Resultant force


  The equivalent single force which could represent all the forces acting on an object
• Resultant force


  Forward Force - Backwards Force
• If no resultant force acts (i.e. the forces are balanced) then the object will either be at rest or travel at a constant speed in a straight line
• If a resultant force acts on the object then it will change speed
• Experiment to design to find how much a spring will stretch when 100g masses are added


  1. Set up apparatus with spring and masses

  2. Measure the unstretched length of spring

  3. Add a 100g mass to the spring and measure the new length

  4. Subtract the original length from the stretched length to get the extension

  5. Repeat adding 100g masses each time
• Gradient


  Change in y / change in x
• The force and the extension are directly proportional (straight line through the origin)
• Hooke's law states that the extension of a spring is directly proportional to the force applied, provided the limit of proportionality is not exceeded
• Spring constant (k)


  Measured in N/cm or N/m, represents the stiffness of the spring
• F=ke

  N/cm