Determine a formula for the acceleration of the system shown in Fig. 4–49 (see Problem 55) if the cord has a non-negligible mass m_C. Specify in terms of ℓ_A and ℓ_B , the lengths of cord from the respective masses to the pulley. (The total cord length is ℓ_A + ℓ_B.)

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Identify the forces acting on the system: The system consists of two masses (m_A and m_B) connected by a cord of mass m_C. The forces acting on the system include the gravitational forces on m_A and m_B, the tension in the cord, and the distribution of the cord's mass.

Account for the cord's mass distribution: Since the cord has a non-negligible mass m_C, its weight is distributed along its length. The effective mass of the cord contributing to the tension on each side of the pulley depends on the lengths ℓ_A and ℓ_B. The mass per unit length of the cord is given by m_C / (ℓ_A + ℓ_B).

Write the equations of motion for each mass: For m_A, the net force is T_A - m_A * g, where T_A is the tension in the cord on the side of m_A. For m_B, the net force is m_B * g - T_B, where T_B is the tension on the side of m_B. The acceleration of both masses is the same due to the cord's constraint.

Relate the tensions to the cord's mass: The tension difference between T_A and T_B arises due to the weight of the cord. The tension at any point in the cord changes linearly with the length. Specifically, T_A = T_B + (m_C * g * ℓ_A) / (ℓ_A + ℓ_B).

Combine the equations to solve for acceleration: Use Newton's second law for the entire system, considering the total mass (m_A + m_B + m_C) and the net force (m_B * g - m_A * g). Substitute the relationship between T_A and T_B into the equations to express the acceleration in terms of m_A, m_B, m_C, ℓ_A, and ℓ_B. The final formula will involve these variables and g.

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Newton's Second Law

Newton's Second Law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This principle is fundamental in analyzing systems involving forces, such as the tension in the cord and the weights of the masses. It provides the basis for deriving equations that relate forces to acceleration in dynamic systems.

Tension in a Cord

Tension is the force transmitted through a cord or rope when it is pulled tight by forces acting at either end. In this problem, the mass of the cord itself contributes to the overall tension, which affects the acceleration of the system. Understanding how tension varies along the length of the cord is crucial for accurately determining the system's dynamics.

Kinematics of a Pulley System

The kinematics of a pulley system involves analyzing the motion of masses connected by a cord over a pulley. The lengths of the cord segments (ℓ_A and ℓ_B) play a significant role in determining how the movement of one mass affects the other. This relationship is essential for deriving the formula for acceleration, as it links the distances moved by the masses to their respective accelerations.