Fundamentals of Physics Extended (10th Edition)

Published by Wiley
ISBN 10: 1-11823-072-8
ISBN 13: 978-1-11823-072-5

Chapter 13 - Gravitation - Problems - Page 385: 99a

Answer

The magnitude of gravitational force, $F=1.5085\times10x^{-12}$ N and its direction is toward the y-axis as shown in figure.

Work Step by Step

Let's take a cartesian coordinate where x and y axes are as shown in the figure. Now let's take a very small piece of the rod $dl$ where $dl \to 0$ and the mass of the piece is $dm_1$. As the distance from the point $P$ and the piece is $R$ then the force for a single piece would be, $$dF = G\frac{dm_{1}m_{2}}{R^2}$$ Here, $G$ is the gravitational constant $m_2$ is the mass of the object at point $P$ But as the rod is in mirror symetry in respect to the y-axis, the x-component of $dF$ would be cancel out by both side. So only the y-component will survive. So now we can write, $$dF = G\frac{dm_{1}m_{2}}{r^2}cos(\theta)\;\; [only\;the\;y-component].....(eq. 1)$$ Now the mass of the rod for unit length is, $$\lambda = \frac{m_1}{\pi R}\\ So, \; dm_1 = \frac{m_1 dl}{\pi R}\\ or, \; dm_1 = \frac{m_1}{\pi R} R d\theta\\ or,\;dm_1= \frac{dm_1 d\theta}{\pi}$$ Now putting this to $eq.1$ we find, $$dF = G\frac{m_1 m_2}{\pi R^2} sin\theta d\theta\\ or, F = G\frac{m_1 m_2}{\pi R^2} \int_{0}^\pi sin\theta d\theta\\ or, F = G\frac{m_1 m_2}{\pi R^2}[-cos\theta]_0^\pi\\ or,\;F=2G\frac{m_1 m_2}{\pi R^2}$$ Now putting values we can find that, $F = 1.5085 \times 10^-12 N\\$ And its direction is toward the y-axis.
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