A wheel of radius 0.20 m is mounted on a frictionless horizontal axis. A massless cord wrapped around the wheel is attaches to a 2.0 kg block that slides on a horizontal frictionless surface inclined at angle θ = 20 degrees with the horizontal. The box accelerates down the surface at 2.0 m/s^(2). What is the rotational inertia of the wheel about the axle? Solution and picture is on page 32-33 http://mrlee.dreamstation.com/apPhysics/lect/apPhysics_lec_10.pdf I understand all the steps except the last one where m1(gsinθ - a1)r2 / a1 / r2 = m1(gsinθ - a1)r2^(2) / a1 why did they divide the denominator a1 by r2? and how did the r2 in the numerator become r2^(2)?? thanks!