Ch.02 – Newton's First Law of Motion – Inertia

2.1 Aristotle on Motion

Aristotle classified motion into two main types, laying the foundation for later developments in physics.

• Natural Motion: Motion of objects that occurs "naturally" (e.g., a stone falling to the ground). Objects move to their proper places based on their nature.

• Violent Motion: Motion imposed on objects by a push or pull force (external influence).

Examples:

• A lump of clay will fall to the ground because it is "natural" for it to do so.

• If a cart is pushed, it will move; when the force stops, it will stop (according to Aristotle).

• Objects that belong "naturally" to the earth will fall, while those that belong to the heavens will rise.

Additional info: Aristotle's view was later challenged by experimental evidence.

2.2 Galileo's Experiments

Galileo conducted experiments to challenge Aristotle's ideas, leading to the concept of inertia.

• Dropped objects of different weights: Galileo showed that objects of different masses fall at the same rate in the absence of air resistance.

• Inclined planes: Galileo rolled balls down inclined planes and observed that they maintain their speed unless acted upon by another force.

Inertia: The property of matter to resist a change in its state of motion. Mass is the measure of inertia.

2.3 Newton's First Law of Motion

Newton refined Galileo's ideas into his first law of motion, also known as the law of inertia.

• Statement: Every object continues in a state of rest or of uniform speed in a straight line unless acted upon by a net external force.

Example: When a tablecloth is skillfully whipped from under dishes on a table, the dishes remain at rest due to inertia.

2.4 Net Force and Vectors

Newton's first law uses the words "net external force." This requires us to define what a force is and how forces combine.

• Force: A push or pull acting on an object.

• Contact Force: Force applied by physical contact (e.g., pushing a chair).

• Long-range Force: Force acting at a distance (e.g., gravity).

Examples:

• Pushing a chair across the classroom (contact force).

• A market falling down (gravity as a long-range force).

Catalog of Forces:

Net Force: The vector sum of all the external forces acting on an object ().

Vector Addition:

Additional info: Scalars have only magnitude, while vectors have both magnitude and direction.

2.5 The Equilibrium Rule

Mechanical equilibrium occurs when the net force acting on an object is zero.

• Static Equilibrium: The object is at rest (velocity is zero).

• Dynamic Equilibrium: The object is moving with constant speed in a constant direction (velocity is constant).

Equilibrium Rule:

Examples of Equilibrium:

• A book resting on a table (static equilibrium).

• A person moving upward at constant speed in an elevator (dynamic equilibrium).