Answer to Question #113470 in Classical Mechanics for Julie Benson

Question

Three children are sitting on a seasaw in such a way that it balances. A 20kg and 30kg boy are on opposite sides at a distance B and 3B , respectively from the pivot. The rod is held horizontal and then released. Find its angular acceleration at the instant it is released.

Answer 1

$$\tau=30g(3b)-20gb=70gb$$

$$I=30(3b)^2-20b^2=250b^2$$

$$\alpha=\frac{\tau}{I}=\frac{70gb}{250b^2}=\frac{70(10)}{250}=2.8\frac{rad}{s^2}$$