# PHYS 2211 Module 5.2

## Newton’s First Law 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! Discuss!

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.