Year 12s: what will be the first way you research your future options? >>

Principle of Moments question - help!

I think I understand the principle of moments, but this question threw me! I've managed to get numbers that look right, but I don't know if they are, and even if I have stumbled across the right answer and method, I don't understand what I did! Help!
Here's the question:

A uniform rectangular beam is supported by two straps. The beam is 4m long, weighs 3600N and is in equilibrium. Strap A is positioned 0.5m from one end of the beam and strap B is positioned 1.0m from the other end.
a) use the principle of moments to show upward force X (the tension in strap A) is 1440N
b) Hence, determine force Y (tension in strap B)

For A, I did 3600/2.5=1440N

For B, I did 1/1.5=0.66667, 1440/0.66667=2160N

Can someone please tell me if I'm right, and if so/not, why???
Thanks :smile:

Reply 1

borysek01

Fort part A) You need to show more working out I think. I'm pretty horrible at Physics, so don't take my word. But this is how I would do it.

I presumed that the beam is uniform (centre of mass acts down the middle, 2m from both ends) and I used Strap B as the pivot.

F1 x D1 = F2 x D2
F1 x 2.5m = 3600N x 1m
F1 = (3600N x 1m) / 2.5
F1 = 1440N

Part B)
Changed the pivot location to A this time and used the same rule.

F1 x D1 = F2 x D2
F1 x 2.5m = 3600N x 1.5m
F1 = (3600N x 1.5m) / 2.5
F1 = 2160N

So yes, you got this right, but the presentation and the numbers were really weird. Good luck! Thanks for the question, it helps me as well to have ab it of practice.