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March 29, 2015

March 29, 2015

Posted by **Annie** on Monday, April 4, 2011 at 11:39pm.

Elizabeth: Since the little brother is half as heavy as her sister, he must sit twice as far from the pivot in order to “compensate” for his lower weight—6 feet instead of 3 feet.

Jill: I used the formula for torque, Torque = Fr, where F is force and r is distance from the pivot. Here, the

relevant forces acting on the see-saw are gravitational, the weights of the children: F1 = 60 pounds and

F2 = 30 pounds. The big sister sits r1 = 3 feet from the pivot, and we’re solving for r2. The torques

must balance: F1r1 = F2r2. I plugged in the numbers to get (60 pounds)*(3 feet) = (30 pounds) * r2, and

solved for r2 to get 6 feet.

1. The two students got the same answer. Did one (or both) of them get lucky, or are both kinds of

reasoning valid? Explain.

2. Are Jill and Elizabeth’s reasoning fundamentally the same, fundamentally different, or a mix of those two extremes? Explain

- Physics -
**drwls**, Tuesday, April 5, 2011 at 12:14amElizabeth used correct logic and was able to do the problem "in her head" by reducing the problem to its essentials. Zero net torque requires equal and opposite torques on opposote sides.

The reasoning of both is fundamentally the same.

- Physics -
**Hana**, Tuesday, November 6, 2012 at 8:36pmhow

- Physics -
**Anonymous**, Tuesday, April 8, 2014 at 2:21pmi dont know

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