Home Physics Motion in a Plane Circular Motion in Vertical Plane A mosquito is sitting on an L.P. record of a…
Physics Motion in a Plane Circular Motion in Vertical Plane Subjective Type
Published on: September 11, 2026

A mosquito is sitting on an L.P. record of a gramophone disc rotating on a turn table at 33 revolution per minute. The distance of the mosquito from the center of the disc is 10 cm. Show that the friction coefficient between the record and the mosquito is greater than 2 / 81. Take g = 10 m/s 2 .

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Verified by Experts
The correct answer is:
A
To determine the friction coefficient between the record and the mosquito, we can use the concepts of circular motion and friction.
Step 1: Calculate the angular speed of the disc in radians per second. Since the disc is rotating at 33 revolutions per minute (rpm), we convert that to radians per second:
\[ \omega = 33 \times \frac{2\pi \text{ rad}}{1 ext{ rev}} \times \frac{1 ext{ min}}{60 ext{ s}} = \frac{33 \times 2\pi}{60} = \frac{11\pi}{10} \text{ rad/s} \]
Step 2: Now, we calculate the centripetal acceleration \(a_c\) of the mosquito at a distance of 10 cm (0.1 m) from the center of the disc using the formula:
\[ a_c = r\omega^2 \]
where \(r = 0.1 \text{ m}\).

So,
\[ a_c = 0.1 \times \left(\frac{11\pi}{10}\right)^2 = 0.1 \times \frac{121\pi^2}{100} = \frac{121\pi^2}{1000} \text{ m/s}^2 \]
Step 3: The frictional force is required to provide this centripetal acceleration; hence, \[ f = ma_c. \]
where \(m\) is the mass of the mosquito (assume a small mass, e.g., 0.001 kg for calculation).

Thus,
\[ f = m \times a_c = 0.001 \times \frac{121\pi^2}{1000} = \frac{121\pi^2}{1000000} ext{ N} \]
Step 4: The normal force \(N\) acting on the mosquito is equal to its weight: \[ N = mg = 0.001 \times 10 = 0.01 ext{ N} \]
Step 5: The coefficient of friction \(\mu\) is given by:
\[ \mu = \frac{f}{N} = \frac{\frac{121\pi^2}{1000000}}{0.01} = \frac{121\pi^2}{10000} \]
Step 6: Now substituting \(\pi\) (approximately 3.14):
\[ \mu = \frac{121(3.14)^2}{10000} \approx \frac{121 \times 9.86}{10000} \approx \frac{1198.66}{10000} \approx 0.1199 \]
Now, to conclude that this is greater than \(\frac{2}{81} \) (which is approximately 0.02469):
Therefore, \(\mu > \frac{2}{81}\) holds true.
Thus, we have shown that the friction coefficient between the record and the mosquito is greater than 2/81. Therefore, the answer is A.

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