# Homework Help: Rotating at a pivot

1. Nov 20, 2008

### Jennifer001

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

A 60-cm-long, 500 g bar rotates in a horizontal plane on an axle that passes through the center of the bar. Compressed air is fed in through the axle, passes through a small hole down the length of the bar, and escapes as air jets from holes at the ends of the bar. The jets are perpendicular to the bar's axis. Starting from rest, the bar spins up to an angular velocity of 150 rpm at the end of 10 s.

How much force does each jet of escaping air exert on the bar?

2. Relevant equations

w1=w0t+1/2alpha(t^2)
f=ma
a=r(alpha)

3. The attempt at a solution

F=ma
a=r(alpha)

so i subbed in a into the force equation

F=m(r(alpha))
and i sub in the velocities to find alpha

w1=1/2(alpha)t^2
15.71=1/2(alpha)(10)^2

alpha=0.3142

sub all the known #s into F=mr(alpha)
F=.5(.3)(.3142)
F=0.04713N

what did i do wrong??

2. Nov 21, 2008

### bowma166

Isn't the equation for angular velocity
$$\omega=\omega_{0}+\alpha t$$?

I think you're confusing it with
$$\phi=\phi_{0}+\omega t+\frac{1}{2}\alpha t^{2}$$