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Classical Mechanics Principles
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Which of the following scenarios best illustrates the Principle of Least Action in classical mechanics?

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Choose the Best Answer

A

A ball rolling down a hill reaches the bottom in the shortest time possible.

B

A pendulum swinging back and forth due to gravitational force.

C

A car accelerating from rest to a maximum speed on a straight road.

D

A satellite orbiting Earth due to gravitational attraction.

Understanding the Answer

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Answer

The Principle of Least Action says that a particle moving from one point to another takes the path that makes the integral of its Lagrangian (kinetic minus potential energy) over time as small as possible. In practice, this means the trajectory satisfies the Euler‑Lagrange equations, which are derived by setting the variation of that action integral to zero. A simple illustration is a ball thrown upward: the ball follows a parabola that makes the action minimal, which is exactly the path predicted by Newton’s laws. Thus, the scenario where a ball follows a parabolic arc from launch to landing best demonstrates the principle.

Detailed Explanation

When the ball rolls down the hill, it follows a path that takes the least time. Other options are incorrect because A pendulum swings because gravity pulls it back and forth, not because it finds the least action path; A car speeding up does not choose a path that minimizes action.

Key Concepts

Principle of Least Action
Newton's Laws
Energy Conservation
Topic

Classical Mechanics Principles

Difficulty

medium level question

Cognitive Level

understand

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