A disk with a radius of 1 mm and a mass of 1 mg rotates around an axis perpendicular to its plane with afrequency of 1 Hz. Calculate the minimal possible value for angle between vector⃗ L and z-axis.
Question
A disk with a radius of 1 mm and a mass of 1 mg rotates around an axis perpendicular to its plane with afrequency of 1 Hz. Calculate the minimal possible value for angle between vector⃗ L and z-axis.
Solution
To solve this problem, we need to understand the relationship between the angular momentum vector (L) and the z-axis. The angular momentum of a rotating object is given by L = Iω, where I is the moment of inertia and ω is the angular velocity.
Step 1: Calculate the moment of inertia (I) The moment of inertia for a disk rotating about an axis perpendicular to the plane of the disk is given by I = 0.5mr^2. Given that the mass (m) is 1 mg = 110^-6 kg and the radius (r) is 1 mm = 110^-3 m, we can calculate I as follows:
I = 0.5 * 110^-6 kg * (110^-3 m)^2 = 0.5 * 10^-6 * 10^-6 kgm^2 = 0.5 * 10^-12 kgm^2.
Step 2: Calculate the angular velocity (ω) The angular velocity is related to the frequency of rotation by the equation ω = 2πf, where f is the frequency. Given that the frequency is 1 Hz, we can calculate ω as follows:
ω = 2π * 1 Hz = 2π rad/s.
Step 3: Calculate the angular momentum (L) Now we can calculate the angular momentum using the equation L = Iω:
L = 0.5 * 10^-12 kgm^2 * 2π rad/s = π * 10^-12 kgm^2/s.
Step 4: Calculate the angle between L and the z-axis The angular momentum vector L is always perpendicular to the plane of rotation. Therefore, if the disk is rotating in the xy-plane, the angular momentum vector L is along the z-axis. Hence, the angle between L and the z-axis is 0 degrees.
So, the minimal possible value for the angle between the vector L and the z-axis is 0 degrees.
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