JULCIT B.

asked • 08/04/16

What is the value of X?

A see saw is exactly in balance.  The weights of the people sitting on it are 50kg,  Xkg and 15kg. They sit at distances of 1.5, 1.5, and 2m from the pivot respectively.  What is the value of X?

Steven W.

tutor
Assuming the seesaw is horizontal, this problem sets up very similar to the previous one you posted.  This time, they suggest an axis of rotation (pivot), and give you the distances from that point, and the board mass appears to be negligible.
 
Try setting it up according to the principles we outlined in your previous post, and see if you can get an answer.  I would be happy to check it with you.
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08/04/16

Steven W.

tutor
The other aspect of this problem that is different from your other one is, the stated pivot point is in the middle, and they do not tell you who is on which side.  Probably just by intuition, you can guess that (unless person X can convince gravity to pull him or her upward) both the X-kg and 15-kg person have to be sitting on one side of the pivot, with the 50 kg person on the other.  Hence, the X-kg and 15-kg people exert torques in the same sense (either + or -) and the 50-kg person exerts a torque in the sense opposite the other two.  Which one(s) is (are) clockwise torques and which one(s) is (are) counterclockwise is immaterial to the solution, as it turns out.
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08/04/16

JULCIT B.

thanks for your assistance. I really appreciate it.
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08/05/16

1 Expert Answer

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Karin S. answered • 08/04/16

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If the seesaw is in balance then clockwise torque equals counterclockwise torque. So torque is force * distance. 
50(1.5)=1.5x + (2)(15)
75=1.5x+30
45=1.5x
x=30 kg
 
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Steven W.

tutor
In terms of calculation, this is exactly correct.  But, just for completeness, it may be worth pointing out that the 50, 1.5, and 15 are not forces; they are masses.  But, to make them into weights, you just multiply each of them by g, and the g in each term can then be cancelled out of the equation.  So it is no harm, no foul.
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08/04/16

Steven W.

tutor
Oops, I meant 50, X, and 15.
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08/04/16

JULCIT B.

Thanks, Karin.
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08/05/16

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