PHYS 2211 Module 5.2

Newton’s First Law

Recommended Reading

5.2 Newton’s First Law

Learning Objectives

By the end of this section, you will be able to:

  • Describe Newton’s first law of motion
  • Recognize friction as an external force
  • Define inertia
  • Identify inertial reference frames
  • Calculate equilibrium for a system

Newton’s First Law — The Law of Inertia

A body at rest remains at rest or, if in motion, remains in motion at constant velocity unless acted on by a net external force.

In other words, Newton’s First Law can be expressed as:

= constant when

When you APPLY forces to objects, the objects respond by accelerating. No force at all means NO acceleration at all. This is NOT obvious to many people! Aristotle (and still many people today) thought you must apply a net force to KEEP an object moving along steadily. But NO, you don’t. Experimentally, that’s 100% incorrect.

Steady motion means no acceleration: no net force is required!

A correct description of nature was discovered by Isaac Newton (1642-1727, England). Here’s a simplified version of Newton’s first law of motion:

If you leave an object alone (no pushes or pulls on it, i.e. no forces applied), then it will not accelerate.

“Not accelerating” means , or , i.e. = constant (same speed, same direction).

An object at rest will remain at rest if no force acts on it. An object in motion remains with the same motion (same direction, same speed) if no force acts on it.

A more careful (and accurate, and useful) statement of Newton’s First Law adds a single new idea:

If there is no net force on an object, the object will not accelerate.

I added the word “net”, that’s the key. There CAN be forces on the object, but as long as they add up (vectorially, ) to zero, then the object will still have .


Practice 5.2.1
An arrow is in mid-flight on its way to a target. At this moment, what force keeps the arrow going forward?
(a) Gravity is pushing the arrow forward
(b) No force is pushing the arrow forward
(c) The force from the bowstring is pushing the arrow forward
(d) Air resistance is pushing the arrow forward
Practice 5.2.2
Somewhere in the universe an object is moving in a slow arcing turn without changing its speed. From just this information, what can we say about the net force on that object?
(a) There cannot be any net force on the object
(b) There must be a net force on the object
(c) There may or may not be a net force on the object, we can’t tell.


Consider how you would answer these questions. Then bring this to class for a group discussion.

Explain the need for automobile seat belts in terms of Newton’s first law.

A skydiver opens her parachute, and shortly thereafter, she is moving at constant velocity. What forces are acting on her? Which force is bigger? Explain.

If an object is at rest, can you conclude that there are no forces acting on it? Explain.