Monthly Mechanics: Weight, Force, and Load

Today’s topic is decoding the language of mechanics. Structural engineers talk a lot about a certain unit of measurement, the one that’s measured in pounds (in the American system) or newtons (in metric). But they have lots of words for it: they might describe a 20-pound load, a 20-pound force, or a 20-pound weight. In all three cases you should just hear, “20-pound weight.” Load, force, and weight basically mean the same thing.

That’s my point for today; feel free to stop reading right now. For some insight into the fine differences between the three words, read on.

Everybody is familiar with the concept of weight. A 20-pound weight is something you might lift at the gym. A toddler could weigh 20 pounds, or a smallish dog. Weight is how hard gravity pulls on an object.

A force is the same thing – pushing or pulling on an object – except it’s not necessarily caused by gravity. Weight is just one of the many types of force. Another type of force is wind. What’s the difference between weight and wind? For one thing, weight pulls you down, whereas wind pushes you sideways. The units are the same (pounds), but a force can happen in any direction, while weight always pulls down.

Weight

Several kinds of forces.

Yet another type of force is a reaction. When my weight pulls me down (action), a normal force from the ground pushes me back up (reaction). When the wind pushes me right (action), a friction force pushes me left (reaction). Newton’s Third Law states that every action has an equal and opposite reaction; all those actions and reactions are examples of forces.

There’s an even finer difference between a force and a load. Force is the actual pushing or pulling, while load is the thing that causes it. For example, “wind load” might be defined as a force of 100 pounds if you’re 10 feet off the ground, a force of 150 pounds if you’re 20 feet off the ground, and so on. “Seismic load” is a force that varies depending on the magnitude and frequency of an earthquake. “Dead load” is the weight of all the parts of a structure that don’t move (floors, walls, roofs), and “live load” is the weight of all the parts that do move (furniture, people).

Weight1

First chapter for wind load in the ASCE 7 code. Most buildings in the US must be designed for the forces described in this code.

If there’s a number attached to it, then strictly speaking it’s a force, not a load. But plenty of engineers use the words interchangeably. That’s why you are safe if any time you hear one of these words, you mentally change it to “weight”.

WeightGraphic

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