What happens if you cut the green rope?

Key Concepts

• Tension: The pulling force transmitted axially by a string, rope, cable, or similar object, or by each end of a rod, truss, or similar three-dimensional structure.
• Slack: Not taut or held under tension; loose.
• Equilibrium: A state in which opposing forces are balanced.
• Force Distribution: How forces are shared among different supporting elements in a system.

Initial Predictions & Observations

The central question posed is: what happens to a weight suspended by a system of springs and ropes if the green rope is cut? Initial predictions from individuals observing the setup overwhelmingly suggest the weight will fall downwards. Several participants explicitly state the weight will “come down,” “drop,” or end up “lower than it started.” One participant predicted a slight movement to the side, but not a significant drop. A key observation made before the experiment is that the red and black ropes are “slack” – meaning they are not currently bearing any of the weight’s load and are therefore not under tension.

System Setup & Force Analysis

The setup consists of two springs connected in series. The upper spring is hung from a hook. The green rope connects the bottom of the upper spring to the top of the lower spring, which in turn supports the weight. This arrangement creates a system where the green rope is actively supporting a portion of the weight’s force. The red and black ropes are present but are intentionally left slack, indicating they are not contributing to the weight’s support. This is crucial because it means the entire weight is initially supported by the green rope and the lower spring.

Experimental Result & Counterintuitive Outcome

Contrary to the widespread prediction, when the green rope is cut, the weight rises instead of falling. This outcome is described as surprising and unexpected. The observation is explicitly stated: “The weight somehow went up.” This directly contradicts the initial hypotheses based on intuitive understanding of gravity and support structures.

Implications & Underlying Principles

The unexpected upward movement demonstrates a misunderstanding of force distribution within the system. The initial assumption was that cutting the green rope would immediately remove support, causing the weight to drop. However, the lower spring was already stretched and exerting an upward force. When the green rope is severed, the lower spring’s stored elastic potential energy is released, causing it to contract and lift the weight. The slack red and black ropes do not immediately become taut and provide support; the initial upward force is solely from the lower spring.

Notable Statement

There are no explicitly attributed quotes beyond the direct observations of participants regarding their predictions ("It will go down," "It drops"). However, the statement "The weight somehow went up" encapsulates the core surprising result of the experiment.

Logical Connections

The video progresses logically from posing a question, gathering initial predictions based on common sense, detailing the experimental setup, presenting the counterintuitive result, and implicitly highlighting the need to reconsider the underlying physics of the system. The initial predictions serve as a baseline against which the actual outcome is contrasted, emphasizing the importance of careful observation and analysis.

Conclusion

The experiment demonstrates that seemingly simple physical systems can behave in unexpected ways. The initial, intuitive prediction of the weight falling was incorrect. Cutting the green rope released the elastic potential energy stored in the lower spring, causing the weight to rise. This highlights the importance of understanding force distribution and the role of stored energy in dynamic systems, rather than relying solely on intuitive assumptions.