2 An amusement park ride consists of a rotating circular platform
3 $d=8.00\U{m}$ in diameter from which $m=10\U{kg}$ seats are suspended
4 at the end of $l=2.50\U{m}$ massless chains (Fig. P5.52). When the
5 system rotates, the chains make an angle of $\theta=28.0\dg$ with the
7 \Part{a} What is the speed of each seat?
8 \Part{b} Draw a free-body diagram of a $m_c=40.0\U{kg}$ child riding
9 in a seat, and find the tension in the chain.
10 \end{problem*} % problem 5.52
14 We will eventually use $v=\sqrt{a_c r}$ as we have in all the other
15 problems in this homework assignment to find $v$.
17 First, we need to find the radius $r$ of the path that the seat takes
20 r=\frac{d}{2} + l \sin \theta
21 =(4.00 + 2.50\sin 28.0\dg)\U{m}
25 Now we need to find the centerward acceleration $a_c$. Drawing a free
26 body diagram of our seat, we see that the only forces acting upon it
27 are the tension \vect{T} and gravity $\vect{F}_g$. We know that the
28 seat does not rise or fall in the vertical (\vect{y}) direction, so
29 summing the forces we have
31 \sum F_y &= T \cos \theta - mg=m a_y=0 \\
32 T &= \frac{mg}{\cos\theta} \label{eqn.T}\\
33 \sum F_c &= T \sin \theta
37 =9.8\U{m/s}^2 \cdot \tan 28.0\dg
38 =5.2108\ldots\U{m/s}^2 \label{eqn.ac}
43 =\sqrt{g \tan \theta \cdot (\frac{d}{2} + l \sin \theta)}
44 =\sqrt{ 5.2108\ldots\U{m/s}^2 \cdot 5.1736\ldots\U{m}}
49 Our free body diagram with a child in the seat will be the same as our
50 diagram from \Part{a} but with a new mass $m'=m + m_c=50\U{kg}$.
52 Before we find the tension in the chain, we should check to see if the
53 chain angle changes. The angular velocity $\omega=v/r$ does not
54 change when people get into the seats (because they are of negligible
55 mass compared to the platform), so we can relate our new velocities
56 $v'$ and $r'$ using the same $\omega$ that we had in \Part{a}.
60 =\frac{\sqrt{ g r \tan \theta}}{r}
61 = \sqrt{\frac{g\tan\theta}{r}}
64 Not that the numerical value is important, just that it is a constant.
65 We can plug $v'=\omega r'$ into our centerward acceleration equation
68 =\frac{\omega^2 r'^2}{r'}
71 And applying this to eqn. \ref{eqn.ac}(which hasn't changed except for
72 the need to substitute primed variables)
74 a_c' &= g \tan\theta'=r' \omega^2 \\
75 g \tan\theta' &= \left( \frac{d}{2} + l \sin\theta' \right) \omega^2
77 The only unknown in this equation is $\theta'$, but the equation is
78 analytically unsolvable. We know $\theta'=28.0\dg$ is one solution,
79 because there are no masses in this equation, so is must also hold for
80 case \Part{a}. Then we have to decide if there will be any other
81 solutions. We know intuitively that any solutions will have $0\dg <
82 \theta' < 90\dg$. Considering the $\sin$ and $\tan$ functions on that
83 interval, we see that $\sin\theta'$ is concave down and continuous
84 over the entire interval, and that $\tan\theta;$ is concave up and
85 continuous over the entire interval. Therefore, the left hand side of
86 this equation only equals the right hand side at a single value of
87 $\theta'$ so our $28.0\dg$ solution is unique. If this doesn't make
88 sense to you, you can graph the right and left hand sides to check.
90 Having proved that $\theta'=\theta$ we can move on to solve for the
91 tension. Using eqn \ref{eqn.T}.
93 T'=\frac{m' g}{\cos \theta'}
94 =\frac{50\U{kg} \cdot 9.8\U{m/s}^2}{\cos 28.0\dg}
98 As far as grading is concerned I will accept anything where you did
101 \item assumed $\theta$ didn't change (skipping the whole $\theta'=\theta$ step)
102 \item assumed $\omega$ didn't change, and you went on to show
103 $\theta'=\theta$ is a valid solution (skipping the uniqueness step).
104 \item assumed $\omega$ didn't change, and proved that $\theta'=\theta$