Page 108 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 4 Forces, Friction, and Torque (Oh My!) 89

Measuring Force
The simplest way to measure force, if you’re trying to weigh something, is to use a
scale. Some scales are mechanical, using weights and springs to turn or balance a dial;
some are electrical. Tools for measuring force come in all different shapes, sizes, and
price ranges. Throughout this book, we’ll use force-measurement tools that are readily
available and affordable.

Mechanical Options
The most affordable option is the standard bathroom scale. This is a smaller version of
the scale you stand on at the doctor’s office. The kitchen scale, its smaller cousin, is
used to measure lighter objects like ingredients for recipes and is more accurate.
These are mechanically based scales, which are easy to use. They typically have a
needle that comes to rest on a dial to indicate the weight. With these scales, the
object pushes on a base to measure a force.

To measure pulling force, you can use a luggage scale or spring scale. You can also
find these at sporting good stores sold as fish scales. Mechanical luggage scales go for
under $10 and look kind of like the scales at grocery stores to weigh produce. You
can purchase spring scales, which are literally just a spring attached to a hook, for
even less. Most spring scales have a housing that indicates the pulling force based on
how much the spring stretches. These generally work only for a small range of forces,
like 5 to 20 lbs. So you need to have a good idea of what you’re measuring before
you choose how to measure it.

Electrical Options
Bathroom and kitchen scales also come in electrical versions. Instead of a system of
springs and levers underneath the platform, these use sensors to detect weight and
display it digitally on a screen. You can use these kinds of sensors directly if you need
to integrate them into a project, but they are not as plug-and-play as the mechanical
options.

• Force-sensitive resistors (FSRs) are used to measure low forces. Their accuracy
is not great (±5%–25%, depending on the application), so they are more
useful for measuring relative weights or as a sensor to indicate whether
something is being squeezed or sat on. An example is the SparkFun
(www.sparkfun.com) sensor SEN-09375, which goes up to 22 lbs.