Question

One of Brian Boitanos 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. Newtons First Law
• E. Newtons Second Law

Answer 1

The correct option is B Conservation of Angular Momentum

Law of conservation of angular momentum states that angular momentum $L$ of an isolated system is always constant ,

if $\tau =0$ then $L=constant$

now $L=I\omega =\Sigma m{r}^2\omega$ ,

where $I=$ moment of inertia ,

$\omega =$ angular velocity ,

$m=$ mass of the body ,

$r=$ perpendicular distance from axis of rotation ,

the given system is also an isolated system so its angular momentum $L$ will also be constant ,when the skater starts rotating with arms closer to his body, $r$ decreases hence $I$ therefore $\omega$ increases to keep the value of $L$ constant i.e. angular velocity increases and he moves faster .