^^ Did vcaa 2011 just then, must say, got that MC question [Question 10 -- Materials].
Only 26% of the state got that right T_T..
Great question, definitely my best moment in physics when that came up, luckily I did my damn project on it (although ironically my partner in that project got it wrong...).
could you please explain that question like why it wouldnt sag? thanks
It's concrete, remember that (I remember a few people using string or something - doesn't work). Between A and B, it would never sag because the centre of gravity is on the outside of B (edit: I'm not sure if that reasoning is correct tbh). Hence, the top of the board between A and B would also be under tension as without the steel, that part would kinda bend upwards.
If you don't believe it, test it with a long ruler (best with a plastic one):
A B
____________________________________
| | ^push down here
You'll notice that the region between A and B rises, even if you hold down A and B as much as possible. Hence the steel is needed at the top.