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Laws of Motion - Aristotle's Fallacy

Grade 11CBSEPhysics

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Aristotle's Law of Motion states that an external force is required to keep a body in motion. According to this view, a body would come to rest if the external force acting on it were removed.

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The fallacy in Aristotle's argument lies in the fact that he failed to account for opposing forces such as friction and air resistance present in the natural world.

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In a real-world scenario, such as a ball rolling on the floor, the ball stops not because 'force is absent', but because the external force of friction opposes its motion.

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To keep a body in uniform motion, an external force is only needed to counter the opposing forces (like friction). If there were no friction, a body in motion would continue to move with a constant velocity vv without any applied force.

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The correct principle is: A net external force is required to change the state of motion (acceleration or deceleration), but the net external force required to maintain uniform motion in a frictionless environment is zero, i.e., ∑Fext=0\sum F_{ext} = 0.

📐Formulae

Aristotle’s Fallacious View: F∝v\text{Aristotle's Fallacious View: } F \propto v

Condition for Uniform Motion: Fnet=Fapplied−Ffriction=0\text{Condition for Uniform Motion: } F_{net} = F_{applied} - F_{friction} = 0

Newton’s Correction: a=Fnetm=0  ⟹  v=constant\text{Newton's Correction: } a = \frac{F_{net}}{m} = 0 \implies v = \text{constant}

💡Examples

Problem 1:

A car is moving on a straight horizontal road at a constant speed of 20 m/s20\text{ m/s}. The engine provides a forward force of 500 N500\text{ N}. According to Aristotle, this force is needed to maintain the speed. What is the actual net force acting on the car, and what happens to the 500 N500\text{ N} force?

Solution:

The net force FnetF_{net} is 0 N0\text{ N}.

Explanation:

Since the car is moving at a constant speed (v=20 m/sv = 20\text{ m/s}), its acceleration aa is 00. According to Newton's First Law, if a=0a = 0, then Fnet=0F_{net} = 0. The 500 N500\text{ N} force provided by the engine is exactly balanced by the resistive forces (friction and air drag). Therefore: Fapplied−Ffriction=500 N−500 N=0F_{applied} - F_{friction} = 500\text{ N} - 500\text{ N} = 0 This disproves Aristotle's claim that a net force is needed for motion; the force is only needed to cancel out friction.

Problem 2:

Suppose an object is moving in deep space, far from any gravitational influence or atmospheric friction. If the object is moving at 100 m/s100\text{ m/s}, how much force is required to keep it moving at this speed?

Solution:

F=0 NF = 0\text{ N}

Explanation:

In an environment with no friction or external resistance, no force is required to maintain uniform motion. This is the heart of the rebuttal to Aristotle's fallacy. The object will continue to move at 100 m/s100\text{ m/s} indefinitely due to its inertia unless acted upon by an external net force.