# Forces: Moment

1. May 18, 2007

### a150daysflood

1. The problem statement, all variables and given/known data

In this problem, take g = 10 m s-2.
A uniform plank of length 14 m long and mass 20 kg is being supported at B and C as shown above.
Two boys X and Y of masses 20 kg and 50 kg respectively are right on top of the supports.
X starts to walk towards A and Y walks towards D at the same time.
If both boys walk at 1.0 m s-1, the plank begins to overturn after how many seconds?

2. Relevant equations
Clockwise moment = anticlockwise moment
fs=fs

3. The attempt at a solution
The answer given is 4seconds.
i can't work it out,really need help.
Need a little hint please.
Thanks.

Last edited: May 19, 2007
2. May 19, 2007

### brett351

re: Moment Problem

Imagine support C is not there. Imagine each boy has walked for 1 second. Each boy exerts a moment around B equal to is weight times his distance from B. If BoyY's moment is larger, the plank will want to rotate CW. But the plank doesn't rotate because support C is there. Will BoyX's moment eventually become larger? If so when?

Now do the same thing imagining that support B is not there.

3. May 19, 2007

### Kenny Lee

Sup flood, this is what I got:

F_y equilibrium gives: B + C = 90g
Moment equilibrium about B gives: t = (C - 60g)/ 7.5g,

Where B and C are the reactions at B and C, and t is the time after X and Y start moving.

Recall that the reactions are given by:

B + C = 90g

I figured that since Y is heavier, the beam will overturn when all the weights are centered at C i.e. when B = 0 and C = 90. Subbing this into the moment eqbm equation, we get:

t = 30g/ 7.5g = 4 s

Which is the answer yea? How'd you get 1 s? Anyhows... yea lemme know if that helped.

................
Kenny

4. May 19, 2007

### a150daysflood

Yea kenny lee your right.
Thanks alot.

Last edited: May 19, 2007
5. May 20, 2007

### Kenny Lee

Glad to be of help =)

6. May 20, 2007

### a150daysflood

But how did you get 7.5g anyway?

7. May 20, 2007

### Kenny Lee

I've attached my working. Its Jpeg. Um you have a look, see if it makes sense. (or if its even right).

G luck

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• ###### working.JPG
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8. May 20, 2007

### a150daysflood

Thanks a zillion for ya effort and time.
God bless..

Last edited: May 20, 2007
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