Free Body Diagram

“Look to your left. Look to your right. These people will not be here when you graduate.” That’s what the dean of engineering told us in the summer of 1996 at Virginia Tech. It wasn’t original. That spiel has been delivered to freshman engineering classes by deans since my dad’s day. And I can only assume they’re still giving that speech. I hope so. It’s one of those things that you take with you.

Harsh / realistic / objective statements and challenges are things you get used to while studying engineering. Your freshman year they try to dissuade you from the profession and that’s when most drop out. The second year they teach you and the third year they try to kill you (thermodynamics, fields, aeronautics, etc). Forth years (and for some of us fifth years or more) are for application of knowledge and blowing things up in the lab. And there’s always time for beer. But I digress.

Free Body Diagrams

One of the most important things I learned my second year of engineering was a concept called Free Body Diagrams. And while my day-to-day activities are a far cry from the evil of Dr Kraig’s Statics class I’ve kept this approach to help me stay sane in an insane world. Leaning to think in this manner has helped me surmount a series of challenges so I thought it best to describe it.

A Free Body Diagram is a picture with a bunch of arrows on it.

Lists and more Lists

There’s also a list of considerations, assumptions, and constants that we may or may not use in this equation. We keep nice and tidy in a list on the side of the page. It helps because we can be a dense bunch. When trying to work some part of a problem we may forget why we chose to interpret something in a certain way. When you get stuck in a solution checking your assumptions usually helps. You may need better assumptions. Or less of them with better data.

Gravity is 9.8 m/s^2. Or we assume that the tires are perfectly round. etc.

The Object

The object is usually generalized as a box. That’s because us left brain engineering types pretty much suck at making pretty pictures. We invented protractors because we can’t draw curves without them. But, hey, sometimes generalizing your issues makes it easier to solve them. Say you’re trying to figure out if a car will roll when headed into a bank. Sure you could spend a lot of time drawing the car but why do that when a box with 4 wheels is really all you need? In fact, every problem we solve gets boiled down to it’s pure essence first.

The Forces

For every force we can imagine acting on that body, an arrow pointing to the place it is applied to on the object is drawn. We note the location of that force and the distance from the center of the object when we think it has meaning. We ascribe a value to each force so we know the relative strengths in comparison to the others in the field. After double checking all of the magnitudes against our assumptions, we disregard the insignificant forces and focus on the real key players.

Then you group like terms to get a good idea of what’s really going on.

By this point we have a box – which is supposed to be a Porsche – or something like it – and there are a bunch of arrows pointing at it with distances from the center. That’s too confusing. So we punt. We engineers like right angles so we resolve everything into an X Axis force and a Y axis force. Got a force going a crazy tangent? We’re going to break it into 2 forces; one X and one Y. Changing new problems into smaller ones that we’re equipped for dealing with makes solving problems easier.

Now all you have to do is cancel out the opposing forces, after accounting for distance from center, and you’re on your way to solving the problem because you have a clear idea of all the angles.

Even better, now that you have a well-ordered problem you can run simulations on what would happen if you changed the existing forces magnitudes or even introduces a new force! (Geeky hurrah here.)

Sure, some problems are thornier than others. But in the end every tractable problem, that is every problem that has a solution, tends to get solved in a similar fashion by following the bolded steps. It doesn’t matter if its a rolling Porsche, a kid on a swing, or trying to achieve your dream. Abstract, identify, sum, and solve. Or Survive, endure, resist, and escape.

This message was brought to you by 5 years of undergraduate engineering education, 150+ credit hours, and Beast Lite.


  1. Hi!
    I’m not certain what disturbs me more: the geek problem-solving manifesto, or the fact that I actually understood it! ๐Ÿ˜€

    Seriously, it is absolutely true, and a good reminder to me to double-check my assumptions *and* to focus on what I want to achieve.

    Looking forward to your triumphant return,


  2. Thanks for coming by, LaVeda! What’s wrong with geeking it out? All the most interesting people are doing it!

    So what is it that you want to achieve? Beside conveying awesome stories over at your site?

    And the posting return commences tonight – inspiration via donuts, espresso, and #AONC_JCC!

  3. Ah Ted…

    Actually, there isn’t anthing wrong with geeking out… It’s one of my favorite things to do! (And we are interesting, aren’t we?)

    I’m still trying to figure out what I want to do when I grow up ๐Ÿ™‚


  4. Why grow up?

  5. Hmmm…

    Good point!

Speak Your Mind