Newton’s First Law of Motion—Inertia

Aristotle's Ideas of Motion

Early understandings of motion were shaped by Aristotle, who classified motion into two types: natural motion and violent motion. Natural motion was thought to be due to the nature of objects seeking their 'proper place,' while violent motion was caused by external forces.

• Natural motion: Objects move straight up or down to reach their natural place (e.g., stones fall, smoke rises).

• Violent motion: Motion resulting from external pushes or pulls (e.g., wind moving a ship).

Galileo's Concept of Inertia

Galileo challenged Aristotle’s views in the 1500s, introducing the concept of inertia. He demonstrated that, in the absence of friction, objects of different weights fall at the same rate and that a force is not needed to keep an object moving.

• Inertia: The property of matter to resist changes in motion; depends on mass.

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

• Galileo’s experiments with inclined planes showed that objects maintain their speed on a frictionless horizontal surface.

Newton's First Law of Motion

Building on Galileo’s work, Newton formulated his First Law of Motion, also known as the Law of Inertia:

• Newton’s First Law: Every object continues in a state of rest or uniform motion in a straight line unless acted upon by a nonzero net force.

Net Force and Vectors

Net Force

The net force is the vector sum of all forces acting on an object. It determines whether and how an object’s motion will change.

• Vector quantity: Has both magnitude and direction (e.g., force, velocity, acceleration).

• Scalar quantity: Has magnitude only (e.g., mass, volume, speed).

• Forces in the same direction add; forces in opposite directions subtract.

Vectors and Resultants

Vectors are represented by arrows, where the length indicates magnitude and the arrowhead shows direction. The resultant is the sum of two or more vectors.

• For vectors at right angles, use the Pythagorean theorem:

• For vectors not at right angles, use the parallelogram rule.

Equilibrium and the Equilibrium Rule

An object is in equilibrium when the net force acting on it is zero. This can be static (at rest) or dynamic (moving at constant velocity).

• Equilibrium rule:

• Forces are vectors; their directions must be considered in the sum.

Support Force (Normal Force)

The support force (or normal force) is the upward force that balances the weight of an object on a surface.

• Example: A book on a table is supported by the table pushing up with a force equal to the book’s weight.

• When standing on two scales, each scale supports half your weight if your weight is evenly distributed.

Equilibrium of Moving Things

Static and Dynamic Equilibrium

Equilibrium can be static (object at rest) or dynamic (object moving at constant velocity). In both cases, the net force is zero.

• Static equilibrium: No motion (e.g., a crate at rest).

• Dynamic equilibrium: Constant velocity (e.g., a crate pushed at steady speed).

The Moving Earth and Inertia

Copernican Revolution and Inertia

Copernicus proposed that Earth moves around the Sun. Critics questioned how objects could move with Earth, but inertia explains that all objects on Earth share its motion.

• When a bird swoops from a branch, it retains Earth’s sideways motion due to inertia.

• If you toss a coin straight up in a moving vehicle, it lands back in your hand because it retains the vehicle’s horizontal motion.

Summary Table: Key Concepts