In an Atwood machine with two masses, which mass accelerates downward?

In an Atwood machine with two masses, which mass accelerates downward?

• A. The lighter mass
• B. The heavier mass
• C. Both masses
• D. Neither mass moves

Answer: (B)

In an Atwood machine, the two masses share the same magnitude of acceleration but move in opposite directions. The heavier mass has a larger weight pulling it downward than the upward pull from the rope tension, so the net force on it points downward. That net downward force causes it to accelerate downward. The lighter mass experiences a net upward force (tension minus its weight), so it accelerates upward with the same acceleration magnitude. The acceleration is a = (m_heavy − m_light) g / (m_heavy + m_light), which is positive downward for the heavier mass. Therefore, the heavier mass is the one that accelerates downward. If the masses were equal, the net force would be zero and neither would move.

In an Atwood machine, the two masses share the same magnitude of acceleration but move in opposite directions. The heavier mass has a larger weight pulling it downward than the upward pull from the rope tension, so the net force on it points downward. That net downward force causes it to accelerate downward. The lighter mass experiences a net upward force (tension minus its weight), so it accelerates upward with the same acceleration magnitude. The acceleration is a = (m_heavy − m_light) g / (m_heavy + m_light), which is positive downward for the heavier mass. Therefore, the heavier mass is the one that accelerates downward. If the masses were equal, the net force would be zero and neither would move.