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Ch 10: Dynamics of Rotational Motion
Young & Freedman Calc - University Physics 15th Edition
Young & Freedman Calc15th EditionUniversity PhysicsISBN: 9780135159552Non è quello che usi tu?Cambia libro di testo
Capitolo 10, Problema 50a

A thin uniform rod has a length of 0.500 m0.500\(\text{ m}\) and is rotating in a circle on a frictionless table. The axis of rotation is perpendicular to the length of the rod at one end and is stationary. The rod has an angular velocity of 0.400 rad/s0.400\(\text{ rad/s}\) and a moment of inertia about the axis of 3.00×10−3kg/m23.00\(\times\)10^{-3}\(\text{kg/m}\)^2. A bug initially standing on the rod at the axis of rotation decides to crawl out to the other end of the rod. When the bug has reached the end of the rod and sits there, its tangential speed is 0.160 m/s0.160\(\text{ m/s}\). The bug can be treated as a point mass. What is the mass of the rod.

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1
Start by understanding the conservation of angular momentum. Since there are no external torques acting on the system, the initial angular momentum of the system (rod + bug) must equal the final angular momentum.
The initial angular momentum (L_initial) is given by the product of the moment of inertia of the rod (I_rod) and its angular velocity (ω_initial). Use the formula: L_initial = I_rod * ω_initial.
When the bug reaches the end of the rod, the system's moment of inertia changes. The final moment of inertia (I_final) is the sum of the moment of inertia of the rod and the moment of inertia of the bug treated as a point mass at the end of the rod. Use the formula: I_final = I_rod + m_bug * L^2, where L is the length of the rod.
The final angular momentum (L_final) is the product of the final moment of inertia (I_final) and the final angular velocity (ω_final). Since angular momentum is conserved, set L_initial equal to L_final: I_rod * ω_initial = (I_rod + m_bug * L^2) * ω_final.
Solve for the mass of the rod (m_rod) using the known values and the relationship between the initial and final conditions. Note that the tangential speed of the bug (v_bug) is related to the final angular velocity by the equation: v_bug = ω_final * L. Use this to find ω_final and substitute back into the angular momentum conservation equation to solve for m_rod.

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Moment of Inertia

Moment of inertia is a measure of an object's resistance to changes in its rotation. It depends on the mass distribution relative to the axis of rotation. For a rod rotating about an axis at one end, the moment of inertia is calculated using the formula I = (1/3) * m * L^2, where m is the mass and L is the length of the rod.
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Intro to Moment of Inertia

Angular Velocity

Angular velocity is the rate of change of angular position of a rotating object, typically measured in radians per second. It describes how fast the object is rotating around a fixed axis. In this scenario, the rod's angular velocity is given as 0.400 rad/s, which helps determine the rotational dynamics of the system.
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Intro to Angular Momentum

Conservation of Angular Momentum

Conservation of angular momentum states that if no external torque acts on a system, its angular momentum remains constant. As the bug moves along the rod, the system's angular momentum is conserved, allowing us to relate the initial and final states to find unknown quantities like the mass of the rod.
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Conservation of Angular Momentum
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