# So what is happening in this angular kinematics equation?

The wheel of a car has a radius of 20 cm. It initially rotates at 120 rpm. In the next minute it makes 90 revolutions. What is the angular acceleration?

So the answer is solved by using one of the angular kinematic equations. More specifically delta theta. The problem I am having trouble understanding is the answer which probably stems from my poor fundamentals.

The answer is: (90x2pi) = 4pi(60) + 1/2 (alpha) (60)^2

So where does 90x2pi come from?

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I'm assuming by "in the next minute" you really mean something like, "after a steady acceleration for one minute it reaches 90 rpm" as otherwise your answer isn't right (and the problem becomes a lot more sophisticated). –  Carl Brannen Jul 21 '11 at 8:01
The equations you're using are equations for $\theta$, the angle through which the wheel rotates. In physics, we measure angles in radians. There are $2\pi$ radians in a circle, so $90$ revolutions is $90*2pi$ radians.