Fake vs. Real Forces

In this post, I am going to talk about real and not real forces as well as the fake centrifugal force (if you don’t like the word “fake” you could replace that with “fictitious”)

First, an example: suppose you are in a car at rest and press the gas pedal all the way down causing the car to accelerate. What does this feel like? If I weren’t skilled in the art of physics, I might draw a diagram something like this:

![Screenshot 20](http://blog.dotphys.net/wp-content/uploads/2008/10/screenshot-20.jpg)

Yes, maybe someone would add gravity and the chair pushing up, but this shows the important points. What is this force of acceleration? What causes this? This is EXACTLY the same thing as centrifugal force. If you think centrifugal force is real, this also should be real. I think this is enough discussion to show that this force (and centrifugal) is not real, but I will continue. There is another mystery: why does it feel like there is a force pushing you back when you accelerate? (if you have read all my blog posts, you may have a hint to the answer).

1-D Motion with a force in Scratch

I can’t remember how I found this, but [Scratch](http://scratch.mit.edu) is a graphical programming language developed at MIT. My kids love this. In order to make sure they don’t know more than I do, I created my own scratch program. I am sure someone from the scratch community will attack it for some reason, but I am ok with that.

The program shows a numerical calculation of the motion of a box with a constant force on it. You change the mass and the force. It “sort of” plots the position as a function of time. Don’t worry python, I still think you are the best.

Basics: Free Body Diagrams

**Pre Reqs:** [Intro to Forces](http://blog.dotphys.net/2008/09/basics-what-is-a-force/), [Vectors](http://blog.dotphys.net/2008/09/basics-vectors-and-vector-addition/)

Hopefully now you have an idea of what a force is and what it isn’t. What do you do with them? The useful thing to do with forces is to determine the total force acting on an object. At the beginning of the introductory physics course, you will likely look at cases where the total force is the zero vector. This is called equilibrium. Even if you are looking at cases where the forces don’t add up to the zero vector (I say that instead of just “zero” to remind you that the total force is still a vector). Physicists like to represent forces on an object by drawing a Free Body Diagram. This is simply a representation of an object and a graphical representation of all the forces acting on that object.

Simply put, in a free body diagram, all the forces acting on the given object are represented as arrows. Let me start with a simple case, a box sitting on a table.

Basics: What is a Force?

**Pre-reqs:** None.

I intend to talk about forces and force diagrams, but there is a more fundamental question to address first. What is a force? Most texts define it as a push or a pull. That really isn’t a bad definition. Maybe a better (or maybe worse) definition would be “forces are things that change the motion of an object” (change being the key word). If I had to choose one definition of force, it would be something like this:

**Force:** *A force is an interaction between two objects. There are 4 known forces:*

• Gravitational force: An attractive long range force between objects with mass
• Electromagnetic force: An attractive or repulsive long range force between two objects with charge
• Strong Nuclear force: An attractive short range force between particles like protons and neutrons
• Weak Nuclear force: A short range force responsible for beta decay. *Yes, I know that is a confusing force – for introductory physics, you won’t use this force*

All forces are some form of the above forces.

**Important properties of forces**

• Forces are an interaction between TWO objects. It is not possible to have a force on an object and not have another object involved.
• Forces are vectors. They have magnitude and direction
• The unit for force is the Newton. If you do a whole bunch of cool stuff, they will name something after you also.
• Forces are NOT properties of an object like mass or speed or color. They are properties of an interaction between two objects. Yes, I already said that, but it is important.

There are some more things about force you will need to know. For now, this should be enough.

MythBusters pulling on a phone book: You are doing it wrong.

The MythBusters aren’t really doing it wrong, but they give me a chance to talk about some physics. In the latest show, they tested the myth that two phone books with their pages alternating were indestructible. To test this, they put the two phone books together and then pulled them apart in a sort of tug of war. Here is a diagram:

Looks great, what is wrong with this? The problem is that by pulling this way, the MythBusters produces 320 pounds of force on the book – but they could have done twice that. This really goes back to the old question: Which would produce a greater tension, two horses pulling in opposite directions, or one horse pulling on a rope tied to a tree. The answer is that both tensions are the same. However, many say that the two horses create a greater tension. The likely thinking in this “two horses are more” answer is that TWO things are doing something must be greater than ONE thing doing something. This reasoning fails because if you tie a rope to a tree, it is doing exactly the same thing the other horse doing: not moving.

Why? A force explanation follows:

How about a massive catapult to replace the space shuttle

I recently saw a comment on a blog somewhere about putting satellites into space (I think it was from a post about a rocket that blew up). The poster suggested using a giant catapult to put things in space instead of rockets. Maybe he or she was kidding, or maybe not. But I have heard this idea before. Would it work?