# Thinking About Forces and Newton's Laws...

BHS -> Mr. Stanbrough -> Physics -> Mechanics -> Newton's First Law -> this page

## Introduction

For the inhabitants of George Lucas' Star Wars universe, survival depended on their understanding and ability to use "THE FORCE". For beginning physics students, survival in mechanics depends on their understanding and ability to use the concept of "force".

Newton's Laws form the core of mechanics. To apply Newton's Laws, you first have to ask "What forces are pushing or pulling on the object in question?" After all, if all of the forces balance (cancel) exactly, Newton's First Law applies - the object will not accelerate. If all of the forces don't balance, Newton's Second Law applies - the object will accelerate. To apply Newton's Laws, the concept of force must be very clear in a physicist's mind.

Typically the beginning physics student learns that:

"A force is a push or a pull."
Hewitt, Paul G., Conceptual Physics, Second Edition, 1992, p. 26

Unfortunately, this simple definition - if not actually wrong - tends to mislead students and cause confusion. An experienced physicist can indeed think of forces as merely "pushes and pulls" - but the experienced physicist has learned to avoid most of the common traps that abound in mechanics. The beginner needs more guidance. Here goes.

## What is a force?

"A force is an interaction between two (material) objects involving a push or a pull."

How is this different from the textbook definition?

First, a force is an "interaction". You can compare a force to another common interaction - a conversation. A conversation is an interaction between 2 people involving the exchange of words (and ideas). Some things to notice about a conversation (or any interaction)are:

1. To have a conversation, you need two people. One person can't have a conversation (although some people try...).
2. A conversation is something that happens between two people. It is not an independently existing "thing" (object), in the sense that a chair is an independently existing "thing".

Forces are like conversations in that:

1. To have a force, you have to have 2 objects - one object pushes, the other gets pushed . In the definition, "(material) objects" means that both objects have to be made out of matter - atoms and molecules. They both have to be "things", in the sense that a chair is a "thing".
2. A force is something that happens between 2 objects. It is not an independently existing "thing" (object) in the sense that a chair is an independently existing "thing".

So, when physicists and engineers say things like "A 20-pound force acts on a box" it sounds like the force is some type of "real" object that does something to the box. Physicists and engineers tend to talk that way, but they know from experience how forces really act. They probably should say something like "I push the box with a force of 20 pounds" since the force is actually an interaction between them and the box. I don't think that scientists are going to change their language, though, even though it causes a lot of conceptual trouble for beginners.

## An Example:

Here is an example of why you have to think clearly and accurately about forces. Suppose that at first you are sitting at rest in a car, then suddenly you push down on the accelerator. What happens? Well, (assuming the engine is running and the car is in gear...) you are "thrown back" in your seat - sometimes quite violently. Why?

Well, you certainly feel pushed. If a force is "any push or pull", then surely a force must be exerted on you. Forces of this type even have a name - they are popularly called "g-forces".

Being a scientist, however, you are not going to be content with glib explanations. Saying "g-force" does not really explain anything - it just gives the situation a name! Naming is not explaining. A scientist would ask "If I am being pushed, something must be pushing me. What?"

### What's Pushing?

Perhaps you think "the acceleration pushes me back". No, accelerations can't push! "Acceleration" is an abstract concept - not a real object. If "acceleration" can push or pull you, then why not other abstract concepts, like "truth", "justice" and "the pursuit of happiness"? Clearly, accelerations can't exert forces.

What about "the car seat is pushing me"? Well, the seat is a real object, and it does exert forces on you - but the seat is behind (and under you) - the seat is pushing you forward (and up) - not back! How could the seat be pushing you toward the back of the car?

What about inertia? Perhaps "inertia is pushing me back"? Sorry, inertia doesn't push anything. Inertia is a property of an object - like its color, size, density, temperature, etc. No, your inertia can't push on you, any more than your armpit odor (another of your properties) can push on you!

Oh, I know! "The air is pushing me! The air causes the g-force!" Well, air is a material object, and it can exert forces on other objects. I'm sure you have seen the damage caused by a hurricane, tornado, or high wind - at least on TV. These tremendous forces are caused by air! But wait a minute! In order for air to exert an appreciable force it has to be moving relatively fast - there has to be a wind - a strong wind. I don't know about your car, but when I press on the accelerator of my car there is NO noticeable wind. Therefore, it isn't the air that is pushing me back into my seat.

It's getting a little desperate here. Maybe "the road pushes me back"? Get serious! The road isn't even touching you - how can it be pushing you?

A little clear thinking can demolish every idea for the source of these "g-forces". If you are going to think of forces as "a push or pull", then you are stuck with the problem that "you can be pushed when nothing is pushing you." This is clearly unpleasant, illogical, unacceptable, silly, and, well, (sorry, I can't help it) pinheaded.

## So, what's really going on here?

First, forces require:

1. a push or pull,
2. an object getting pushed or pulled,
3. and an object doing the pushing or pulling

Since there is no object pushing you back into your seat (remember, your seat pushes you forward...), requirement #3 is clearly not met. Therefore, there is NO FORCE pushing you back into your seat. (Sure, you feel pushed - so what? Being pushed does NOT mean that a force is acting! Forces are NOT just "a push or pull"!)

## The correct explanation is:

"You are at rest, and you want to remain at rest (Newton's First Law). The car accelerates, and you don't - until something (your seat) exerts an unbalanced (real) force on you, causing you to accelerate."

That's all there is to it.

"G-forces", "centrifugal forces" and their ilk are NOT REAL FORCES. They do not, have not, and never will push on anything! Physicists call these beasts "fictitious forces" - they are fiction - they feel like pushes and pulls, but they don't really exist as forces.

## Moral of the story:

Beware fictitious forces!

Any time you think "a force is acting" - stop. Ask yourself "What object is doing the pushing/pulling here?" If you can't clearly identify the object doing the "forcing", there probably isn't any force. Start thinking about inertia. (Remember - inertia is NOT a force...)

## P.S.

If you aren't totally disoriented by now, you may be thinking "Did he really answer the question - what is a force really?". Well, no. I didn't. There are 2 reasons for that:

1. You don't really need to know what a force "really is" to use forces intelligently in mechanics, because
2. There is considerable disagreement among physicists as to what a force "really is".

This is not the place to go into this deeply, but here's the argument...

If you are a quantum physicist, dealing with the subatomic realm, a force is an exchange of virtual particles. For example, an electromagnetic force is brought about by the exchange of "virtual photons". Other forces are "mediated" by other particles, sometimes called "bosons".

If you are a physicist dealing with general relativity, dealing with the realm of the very large and massive, then a force is a curvature of spacetime. For example, the earth orbits the sun because the sun distorts spacetime in its vicinity, and the Earth responds to the curvature of spacetime at the Earth's location. The force of gravity is a curvature of spacetime.

These different viewpoints both work quite well in their respective realms - but no one to date has shown a connection, or any common ground, between the two concepts of force. There must be one - or else there is some other explanation that includes, in some way, both of these ideas. Why don't you be the one to resolve this problem?

May the "force" be with you.

See also Riding in a Car and the lab activity Newton's First Law in a Car, as well as What is a Force, Really?.