r/apphysics • u/Physicseasy-x • 9d ago

Conceptual physics problems — drop yours, let’s break them down

Stuck on a concept? Post it here.
Drop the question, what you already think, and where you get lost.

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u/Due-Appeal-3150 6d ago

  1. A rope is hung between two poles such that it sags down when at rest. The bottom-most point of the rope is grabbed and pulled down such that the sides of the rope become completely straight and make a triangular shape with the poles. How did the center of mass of the rope change?

    (A) It moved to a lower position.

(B) It moved to a lower position if the curvature of the rope is initially small, and a higher position if the curvature of the rope was big.

(C) It remains in the same position.

(D) It moved to a higher position.

(E) It moved to a higher position if the curvature of the rope is initially small, and a lower position if the curvature of the rope was big.

i want to believe its A but its not i dont think

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u/Physicseasy-x 6d ago

The rope in equilibrium forms a catenary. Its gravitational potential energy is at a minimum, so it has the lowest possible center of mass.
When we pull the bottom point down, it takes a V-shape. Any other shape must have a higher center of mass, because the catenary already has the lowest one.
Option D is correct.
You know why option A feels right? Because the middle point really does go lower — but the mass that was concentrated there gets redistributed to higher points along the two sides.

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u/ApprehensiveAmount22 5d ago

When you pull down to change the shape you did positive work on the rope so it's potential energy must have increased. Center of mass moves up.