Introduction

Newton's first law introduces the force concept in relation to an object's speed and direction. If an object is in motion or at rest, the only way for the object's speed or direction to change is for an external force to act upon it. For example, a stationary ball in outer space will only begin to move when a force is applied to the ball. Since the ball begins to move we can say that its velocity has changed. Additionally, a ball in motion has no reason to change its velocity, speed or direction, unless an external force acts upon it.

   

Key Terms

Text/Videos

Here is a concise explanation of Newton's first law;

Here is the PhysClips section on Newton's First Law. Revisit these clips for the other two law. Note: This professor uses an underline notation for vectors;

• Watch Doc Schuster's inertia video for further explanations about Newton's 1st Law;

• This educational film from 1960 called Frames of Reference discusses/illustrates a very important part of Newton's Laws and will give you a very good "intuition" on inertial reference frames;

• This page on PhysClips discusses many aspects of Newton's Law and most importantly to you the limitations of the Laws.
• Here is an interactive page on EasyPhysics which summarizes Newton's Laws nicely as well as providing questions for you to answer. 
• An Astronomer answers questions about Newton's 1st Law with the H.R MacMillan Space Center;