(a) Starting from I = r² dm show that a thin rod, length L, mass M, spinning about an axis at one end has a moment of inertia of I = ML² /3. (b) If such a rod (initially stationary) has a mass of 3 kg, a length of 0.8 m, and is spun up by a torque of 100 N m acting for 10 s, how fast is it now spinning? What is its angular momentum? (c) A second rod is similar to the first, but has a mass per length, A, which is zero at the axis of rotation but rises linearly to λ = 6 kg m-¹ at the other end. Find I for this second rod. (d) If this second rod is spun up by the same torque for the same time as the first rod, what is its angular momentum?
Angular Momentum
The momentum of an object is given by multiplying its mass and velocity. Momentum is a property of any object that moves with mass. The only difference between angular momentum and linear momentum is that angular momentum deals with moving or spinning objects. A moving particle's linear momentum can be thought of as a measure of its linear motion. The force is proportional to the rate of change of linear momentum. Angular momentum is always directly proportional to mass. In rotational motion, the concept of angular momentum is often used. Since it is a conserved quantity—the total angular momentum of a closed system remains constant—it is a significant quantity in physics. To understand the concept of angular momentum first we need to understand a rigid body and its movement, a position vector that is used to specify the position of particles in space. A rigid body possesses motion it may be linear or rotational. Rotational motion plays important role in angular momentum.
Moment of a Force
The idea of moments is an important concept in physics. It arises from the fact that distance often plays an important part in the interaction of, or in determining the impact of forces on bodies. Moments are often described by their order [first, second, or higher order] based on the power to which the distance has to be raised to understand the phenomenon. Of particular note are the second-order moment of mass (Moment of Inertia) and moments of force.
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