The Equations of Rotational Kinematics

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pstfleur
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1. A spinning wheel on a fireworks display is initially rotating in a counterclockwise direction. The wheel has an angular acceleration of -4.00rad/s2. Because of this acceleration, the angular velocity of the wheel changes from its initial value to a final value of -25.0rad/s. While this change occurs, the angular displacement of the wheel is zero. Find the time required for the change in the angular velocity to occur.



2. W=Wo+xt
Displacement=1/2(W+Wo)t
DIsplacement=Wot+1/2xt^2




3. The answer is suppose to be 12.5s..
OK the initial velocity is what I don't have, i know that its going to be negative.. In order for me to get the Initial angular velocity, i need the Time, which I also don't have.. I don't know how to answer with two unknowns.
 
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bumping again for the 2nd day :)