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Monthly Mechanics: Actions and Reactions

As construction slows down for the winter, it’s hard to keep the blog strong, so I’ll inject it with a monthly shot of structural mechanics. This IS Engineer Unplugged, after all. Today’s post begins a series of columns designed to help you think like an engineer.

Newton’s Third Law states that every action has an equal and opposite reaction. You’re probably thinking of executive ball toys and car collisions, but the law holds true throughout nature, even on objects that aren’t moving. When you stand in place, your weight pushes down on the floor; that’s the action. The reaction is the floor pushing back up on you with a force equal to your weight.

For every action, there is an equal and opposite reaction.

For every action, there is an equal and opposite reaction.

Think about that for a minute if it doesn’t make sense. If you stand on a thick carpet, the carpet smooshes down because it’s too weak to push back up with equal force. An ant could stand on the same carpet without it smooshing down at all. Since the ant doesn’t weigh much, the action is small and the reaction is equally small, small enough for the carpet to sustain. And by the way, you’re in no danger… below the carpet is a subfloor plenty strong enough to push back up on you.

The real fun begins when you step on a beam, because you can trace the reactions further. A beam is a long piece of wood, or steel, or plastic – anything, really – suspended in the air between two supports. Start looking for beams around you and you’ll find them everywhere: a ladder rung, a log bridging over a stream, a car held up by its axles. If you’re standing on one, the beam carries your weight, and reactions occur at the supports. The size of each reaction depends on how close you are to one support, but the two reactions always add up to equal your weight… plus the weight of the beam.

A picture is worth a thousand words.

A picture is worth a thousand words.

Terry standing on a real-life beam.

Terry standing on a real-life beam.

Just about every engineering problem begins with calculating reactions, whether it’s a mile-long bridge or a lowly doorway header. I hope I gave a clear explanation for folks who don’t have a math-y background, and a fun refresher to those who have learned this concept before.

If you can’t explain it to a six year old, you don’t understand it yourself.
-Albert Einstein


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