# 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:

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:

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

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

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

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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