# Rotational kinematics - need some help

1. Jun 25, 2008

### HT3

Rotational Kinematics HELP!?

I desperately need help with two rotational kinematics problems. I do not know what to do in both cases. If someone could be kind enough to go through a solution for me to see it would be very helpful in understanding the problem. Please try and be detailed in your answer as I really want to understand how it is solved.

1. The problem statement, all variables and given/known data

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.
Answer: 12.5s

2) A fan blade whose angular acceleration is a constant 2.00 rad/s^2 rotates through an angle of 285 radians in 11.0s. How long did it take the blade starting from rest to reach the beginning of the 11.0s interval?
Answer: 7.45s

2. Relevant equations
newtonian rotatational formulae.

3. The attempt at a solution
no idea i did the last 20 problems no problems but these 2 are stumping me.

2. Jun 25, 2008

### Kurdt

Staff Emeritus
3. Jun 26, 2008

### physixguru

These hints should be enough:

1).negative acceleration means deceleration.
zero angular displacement means one complete revolution.
counter clockwise is positive direction.

2).2 pi radian = 1 revolution.
calculate the distance traversed in the desired time interval.

4. Jun 26, 2008

### JaWiB

I don't see how there's enough information there to answer the question. Specifically, what is the initial angular velocity?

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