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Age 16 to 18 Challenge Level:

Newton, from Macquerie Fields primary school thinks that the bridges of even length will be the most stable and not feel any forces at the joints. Is he right? Can you use vectors and mechanics to help?
Fygliwu presents a very intuitive mechanical explanation:

A bridge of even length should be less stable due to the fact that it has a central point which bears a load. On a bridge of uneven length this load is shared between a set of three points, the two in the middle on the bottom and the one in the middle on the top. This therefore means that the load of an uneven bridge is spread out and therefore dissapated whilst the load of an uneven bridge in focused on a single joint.  
Can anyone supply mathematical justification?