CIE IGCSE Physics

Revision Notes

1.5.7 Centre of Mass

Finding the Centre of Mass

  • The centre of mass of an object (sometimes called the centre of gravity) is the point through which the weight of that object acts
  • For a symmetrical object of uniform density (such as a symmetrical cardboard shape) the centre of mass is located at the point of symmetry:

, IGCSE & GCSE Physics revision notes

The centre of mass of a regular shape can be found by symmetry

 

  • When an object is suspended from a point, the object will always settle so that it’s centre of mass comes to rest below the pivoting point
  • This can be used to find the centre of mass of an irregular shape:

rotation-force, IGCSE & GCSE Physics revision notes

Diagram showing an experiment to find the centre of mass of an irregular shape

 

  • The irregular shape (a plane laminar) is suspended from a pivot and allowed to settle
  • A plumb line (lead weight) is then held next to the pivot and a pencil is used to draw a vertical line from the pivot (the centre of mass must be somewhere on this line)
  • The process is then repeated, suspending the shape from two different points
  • The centre of mass is located at the point where all three lines cross

 

Stability

  • An object is stable when its centre of mass lies above its base

 

Stability, IGCSE & GCSE Physics revision notesStability, IGCSE & GCSE Physics revision notes

The object on the right will topple, as its centre of mass is no longer over its base

 

  • If the centre of mass does not lie above its base, then an object will topple over
  • The most stable objects have a low centre of mass and a wide base

 

Stability 2, IGCSE & GCSE Physics revision notes

The most stable objects have wide bases and low centres of mass

 

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