# Homework Help: Rotational Dynamics

1. Jan 30, 2008

### Lyphta

1. The problem statement, all variables and given/known data
A wheel initially rotates counterclockwise about a fixed axis, with an initial angular speed of 1.2 radians per second. If it accelerates uniformly to 2.0 radians per second clockwise during a 5 second interval, what is the magnitude of its displacement during this interval?

2. Relevant equations
$$\omega$$f$$^{2}$$ - $$\omega$$i$$^{2}$$ = 2$$\theta$$$$\alpha$$

3. The attempt at a solution
$$\omega$$i = 1.2 rad/sec
$$\omega$$f = 2 rad/sec
t= 5 sec
$$\theta$$ = ?

2^2 - 1.2^2 = 2 $$\theta$$ ... But I found out I cant use that equation, thats where I've been stumped at...

1. The problem statement, all variables and given/known data
A flywheel initially spinning at 1800 rpm is brought to rest in 3 minutes. How many revolutions does the flywheel make in coming to rest?

2. Relevant equations
$$\omega$$f$$^{2}$$ - $$\omega$$i$$^{2}$$ = 2$$\theta$$$$\alpha$$

3. The attempt at a solution
$$\omega$$i = 1800 rpm
$$\theta$$ = ?
$$\omega$$i = 0
$$\alpha$$ = -3769.9 rad/min^2
t= 3 minutes.

0 - [(1800)(2$$\pi$$) = 2(-3769.9)($$\theta$$
but i dont end up with 2700 rev....

Last edited: Jan 30, 2008
2. Jan 31, 2008

### Shooting Star

Do you know any other eqn apart form this? There are three major eqns for uniform acccn, whehter linear or angular. Find alpha from one of those, and then find theta.