2007 AL Physics mc Q1

The answer is option D

To establish equilibrium, the direction of forces on rods P and Q shall not be the same. This excludes options B and C

Option A is impossible, as rod P would not be in compression.

In option D, just image that if the weight fell down, rod P would be extended and rod Q would be compressed.

2013-03-24 21:57:55 補充：
If the directions of rods P and Q were the same, this would give all left or all right force components. The set up could not be in equilibrium.

2013-03-24 22:23:35 補充：
Next, imagine that if rod Q is not present, then there would be two forces at the hinge, one is the normal reaction givenby the hinge (this acts horizontally to the right), and the other is acting acting vertically upward. Such upward force would be balanced by the weight W.

2013-03-24 22:27:06 補充：
(cont'd)... thus the presence of Q is to provide a left-pointing horizontal force to balance the right-pointing force of Q. A left-pointing force wold mean that Q is in compression.
As there is no numerical figures given in the problem, it cannot be explained by claculations.

2013-03-24 22:28:55 補充：
sorry.. a typing mistake in the last post, it should read:
... thus the presence of Q is to provide a left-pointing horizontal force to balance the right-pointing force of P. A left-pointing force wold mean that Q is in compression.