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

Here’s how two different students approached part A.

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 12:14am
Elizabeth 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 8:36pm
how

- Physics -
**Anonymous**, Tuesday, April 8, 2014 at 2:21pm
i don't know

- Physics -
**lorian Bowen**, Wednesday, November 11, 2015 at 10:26pm
both are the same

- Mathematics -
**Rachna was given three natural number and told to add them up rishi was given the same three number and told to multiply them . Surprise rachna and rishi got the same answer. What number were they given?**, Saturday, June 25, 2016 at 2:16am
What is answers for this question.

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