Introduction to SAT II Physics

momentum will be tested in a conceptual manner on SAT II Physics.
EXAMPLE

One of Brian Boitano’s crowd-pleasing skating moves involves initiating a spin with his arms extended

and then moving his arms closer to his body. As he does so, he spins at a faster and faster rate. Which

of the following laws best explains this phenomenon?

(A)Conservation of Mechanical Energy

(B)Conservation of Angular Momentum

(C)Conservation of Linear Momentum

(D)Newton’s First Law

(E)Newton’s Second Law

Given the context, the answer to this question is no secret: it’s B, the conservation of angular
momentum. Explaining why is the interesting part.
As Brian spins on the ice, the net torque acting on him is zero, so angular momentum is
conserved. That means that I is a conserved quantity. I is proportional to R^2 , the distance of the
parts of Brian’s body from his axis of rotation. As he draws his arms in toward his body, his mass
is more closely concentrated about his axis of rotation, so I decreases. Because I must remain
constant, must increase as I decreases. As a result, Brian’s angular velocity increases as he
draws his arms in toward his body.

Key Formulas

Angular

Position

Definition of

a Radian

Average

Angular