5.17 - Newton's 1st & 2nd Laws

Overview

This lecture explains Newton's First and Second Laws of Motion, discusses circular motion and inertia, and shows how forces relate to acceleration.

An object's motion will not change unless acted on by a resultant (net) force.
If the resultant force on a stationary object is zero, it remains stationary.
If the resultant force on a moving object is zero, it continues moving at the same velocity.

A non-zero resultant force causes an object to accelerate.
Acceleration can start motion, increase speed, slow down, stop motion, or change direction.
Acceleration is defined as change in velocity per change in time (velocity includes both speed and direction).
Force and acceleration are directly proportional: doubling the force doubles the acceleration.
The equation ( F = m \cdot a ) relates force (F), mass (m), and acceleration (a).
Example: For mass 0.25 kg, rightward force 42 N, leftward force 30 N, resultant force = 12 N right; acceleration = ( \frac{12}{0.25} = 48 , \text{m/s}^2 ).

In circular motion, if speed is constant but direction changes, the object is still accelerating.
Example: The Moon orbits Earth at constant speed, but changes direction due to Earth's gravitational pull (acceleration towards Earth).

Inertia: an object's tendency to keep its motion unchanged unless acted on by a force.
Inertial mass measures how hard it is to change an object's velocity and is calculated by dividing force by acceleration (( m = \frac{F}{a} )).
Larger masses have greater inertia and require more force to change velocity.

Resultant Force — The single force that represents the vector sum of all forces acting on an object.
Acceleration — The rate of change of velocity over time.
Inertia — The tendency of an object to resist changes in its motion.
Inertial Mass — A measure of how much an object resists acceleration, found by dividing force by acceleration.
Velocity — Speed in a given direction.