1. ## calculating moments (torque)?

Can someone please show me how to do this question, thank you

the diagram shows a balance beam with two gymnasts standing on it, labelled A and B. The balance beam has a mass of 40kg and is assumed to be uniform. Gymnast A weighs 35kg and gymnast B weighs 30kg. Determine the reaction at each support.
I have drawn the picture from my book on paint, hopeing somebody can make sense of my picture.

2. Take advantage of the fact that a force acting on the pivot produces zero torque.

Let the reaction forces be labelled $R_1$, $R_2$ and they are acting upwards. The system is assumed to be in equilibrium, consequently

$\tau_{\text{net}}=0$

Pivot at $R_1$, and then find $R_2$ using the equation:

$\tau_{\text{clockwise}}=\tau_{\text{anti-clockwise}}$

After finding $R_2$, pivot at $R_2$ and then find $R_1$ using the same equation:

$\tau_{\text{clockwise}}=\tau_{\text{anti-clockwise}}$

3. Originally Posted by Chop Suey
Take advantage of the fact that a force acting on the pivot produces zero torque.

Let the reaction forces be labelled $R_1$, $R_2$ and they are acting upwards. The system is assumed to be in equilibrium, consequently

$\tau_{\text{net}}=0$

Pivot at $R_1$, and then find $R_2$ using the equation:

$\tau_{\text{clockwise}}=\tau_{\text{anti-clockwise}}$

After finding $R_2$, pivot at $R_2$ and then find $R_1$ using the same equation:

$\tau_{\text{clockwise}}=\tau_{\text{anti-clockwise}}$
I dont understand what you saying? Can you like start me off ?

4. This is what I have done so far, but I don't get the right answer.

$3R_{2} + 35 \times 1= 1.5 \times 40 + 4 \times 30$

can someone show me whats wrong with my equation?

thank you!

btw I redrew my picture

5. Originally Posted by Tweety
This is what I have done so far, but I don't get the right answer.

$3R_{2} + 35*1= 1.5 * 40 + {\color{red}1}* 30$

can someone show me whats wrong with my equation?

thank you!

btw I redrew my picture
Everything looks right (assuming that the LHS are the counterclockwise torques, and the RHS are clockwise)

However, the number in red should be 4, because the 30kg mass is 4 meters away from the pivot (p1) [Do you see why?].

6. Originally Posted by Chris L T521
Everything looks right (assuming that the LHS are the counterclockwise torques, and the RHS are clockwise)

However, the number in red should be 4, because the 30kg mass is 4 meters away from the pivot (p1) [Do you see why?].

oh yes sorry that was a typo, I meant to put 4 meters.

my equation can't be right cause I don't get the right answers, the reactions forces R1 and R2 equal 474N and 555N. thats the answers given by my book ?

7. Originally Posted by Tweety
oh yes sorry that was a typo, I meant to put 4 meters.

my equation can't be right cause I don't get the right answers, the reactions forces R1 and R2 equal 474N and 555N. thats the answers given by my book ?
Ah...that's it! You're answer is in kilograms, not newtons. To fix that, multiply your answers by 9.8 and you should get the desired answers.

8. Originally Posted by Chris L T521
Ah...that's it! You're answer is in kilograms, not newtons. To fix that, multiply your answers by 9.8 and you should get the desired answers.
Oh right!! I dont believe I missed that! thanks a lot for you help.