1 \begin{problem*}{10.44}
2 A space station is constructed in the shape of a hollow ring of mass
3 $m = 5.00\E{4}\U{kg}$. Members of the crew walk on a deck formed by
4 the inner surface of the outer cylindrical wall of the ring, with a
5 radius of $r = 100\U{m}$. At rest when constructed, the ring is set
6 rotating about its axis so that the people inside experience an
7 effective free-fall acceleration equal to $g$. The rotation is
8 achieved by firing two small rockets attached tangentially at opposite
9 points on the outside of the ring.
10 \Part{a} What angular momentum does the space station acquire?
11 \Part{b} How long must the rockets be fired if each exerts a thrust
13 \Part{c} Prove that the total torque on the ring, multiplied by the
14 the time interval found in \Part{b}, is equal to the change in angular
15 momentum found in \Part{a}. This equality represents the
16 \emph{angular impulse-angular momentum theorem}.
17 \end{problem*} % problem 10.44
21 The certerward acceleration of people on the wall of the space station
24 g = a_c &= \frac{v^2}{r} = r \omega^2 \\
25 \omega &= \sqrt{\frac{g}{r}}
27 Where we used $v = r\omega$ to replace the linear velocity $v$. The
28 moment of inertia of a ring is given by $I = mr^2$ from table 10.2 on
29 page 300. The angular momentum is then given by
31 L = I \omega = m r^2 \sqrt{\frac{g}{r}} = \ans{m r \sqrt{gr}} = \ans{1.57\E{8}\U{Js}}
35 The torque on the station is given by
37 \sum \tau &= 2 \cdot r \cdot F = I \alpha = m r^2 \alpha\\
38 \alpha &= \frac{2 F}{m r}
40 Going back to our constant acceleration equations, we see that
42 \omega &= \alpha t + \omega_0 = \alpha t \\
43 t &= \frac{\omega}{\alpha} = \sqrt{\frac{g}{r}} \cdot \frac{m r}{2 F} = \sqrt{g r} \frac{m}{2F}
44 = \sqrt{ 9.80\U{m/s}^2 \cdot 100\U{m}} \frac{5\E{4}\U{kg}}{2 \cdot 125\U{N}}
45 = \ans{ 6.26\U{ks} = 1.74\U{hr} }
50 \tau t &= I \alpha t = I \omega = L \\
51 2 r F t &= 2 \cdot 100\U{m} \cdot 125\U{N} \cdot 6.26\E{3}\U{s}
54 So they are equal both symbolically and numerically which means I
55 probably didn't make any algebra mistakes (we can hope).