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

## Answer this Question

## Related Questions

- physics - A brother (mass 20 kg) and his big sister (mass 32 kg) want to play ...
- Physics - A 450.8 N child and a 101.0 N child sit on either end of a 2.2 m long ...
- Physics - A 450.8 N child and a 101.0 N child sit on either end of a 2.2 m long ...
- physics - A playground seesaw is exactly 3.00 m long. A little girl who weighs ...
- Physics - A uniform plank weighing 50N and 12m long is pivoted at a point of 4m ...
- physics - A uniform plank weighing 50N and 12m long is pivoted at a point of 4m ...
- Physics - Ali and hid father sit at the ends of a see-saw, 2 m from the pivot, ...
- Physics - Ali and his father sit at the ends of a see-saw, 2 m from the pivot, ...
- English - 1. Sister gave me a pen. 2. My sister gave me a pen. 3. Brother handed...
- Physics - 1-A car is lifted a certain distance in a service station and ...