Andre from Tudor Vianu National College,
Bucharest, Romania produced a good solution to this question. Can
you build on these ideas to answer the extension parts of the
The net force on the horizontal axis must be zero at each pin
joint, so the projections of the forces on X and Y on the
horizontal axis must have opposite directions. So, even if we do
not know their magnitude we can still determine whether they are in
compression or in tension.
If X is under tension then we must have:
The forces must be symmetrical in respect to a vertical axis which
passes through the central weight. Starting from the left toward
the centre, I obtained the following type of forces throughout the
For the second part, a similar analysis shows that the structure
must be as follows:
There would be a net downward force at P, so the equilibrium would
not be stable.