# Homework Help: 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.

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?

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?