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# Mechanics question! watch

1. Jack and Jill are playing on a see-saw made from a uniform plank AB of length 5 m pivoted at M, the mid-point of AB. Jack has mass 35 kg and Jill has mass 28 kg. Jill sits at A. Where must Jack sit for the plank to be in equilibrium when horizontal?

Let d = distance from M to Jack
R1 = Reaction force for Jill
R2 = Reaction force for Jack

What I done:

0 = (28g x 2.5) + (R2 x d) - (35g x d) - (R1 x 2.5)

28g + 35g = R1 + R2

I dont how to find out d!
Help
2. (Original post by Seatbelt)

0 = (28g x 2.5) + (R2 x d) - (35g x d) - (R1 x 2.5)
What was the basis for this equation
3. Sum of clockwise = Sum of anti-clockwise, I think
4. (Original post by Seatbelt)
Sum of clockwise = Sum of anti-clockwise, I think
If you are taking moments you need to do so about a point

what point did you use
5. I used M, thats the only point which you can take moments in right??
6. (Original post by Seatbelt)
I used M, thats the only point which you can take moments in right??
7. I think the important thing is that you don't need to be considering the reaction forces R1, R2. These are not forces acting on the plank.

The plank is presumably stationary when horizontal and in equilibrium.
8. (Original post by TenOfThem)
Why would that be the only point?

I would have taken moments about A
why?? I thought moments only act on pivots?
9. (Original post by ghostwalker)
I think the important thing is that you don't need to be considering the reaction forces R1, R2. These are not forces acting on the plank.

The plank is presumably stationary when horizontal and in equilibrium.
If I exclude R1 and R2 then I've no idea how to work it out, my methods so far is based on what my teacher had done in lesson
10. (Original post by Seatbelt)
If I exclude R1 and R2 then I've no idea how to work it out, my methods so far is based on what my teacher had done in lesson
If you leave out R1 and R2 in your first equation, then you only have one unknown "d".

0 = (28g x 2.5) - (35g x d)
11. Just like ghostwalker said; the reaction forces R1 and R2 are unnecessary to consider in this question.
You know how far Jill sits from the pivot and the force she applies at that point. You know the force Jack would provide as you know his mass, you just don't know the distance.
It's then a case of anticlockwise moments = clockwise moments with only 1 unknown
12. (Original post by ghostwalker)
If you leave out R1 and R2 in your first equation, then you only have one unknown "d".

0 = (28g x 2.5) - (35g x d)
I dont understand why we can exclude the reaction forces, according to my teacher there's always a reaction force acting on an object if it is touching another object?
13. (Original post by Seatbelt)
I dont understand why we can exclude the reaction forces, according to my teacher there's always a reaction force acting on an object if it is touching another object?
The reaction forces is acting on Jack and Jill (and stopping them from falling to earth with a bump); they're not acting on the plank.
14. what I done

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15. (Original post by Seatbelt)
what I done

Posted from TSR Mobile
As has been said those R values do not act on the plank so they are not included when you take moments
16. The answer in the book is 0.5
0 = (28g x 2.5) - (35g x d)
From this equation I get d=2??
17. (Original post by Seatbelt)
The answer in the book is 0.5
0 = (28g x 2.5) - (35g x d)
From this equation I get d=2??
Depends how the question/answer is phrased.

2m from M is 0.5m from B.

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