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118 Making Things Move

this space, and a couple innovative products are currently being tested. Bionic Power
(www.bionic-power.com) created a knee brace with a generator in it that the company
claim generates 7 watts of electricity per leg when walking. It’s generated as your leg
swings in the air, so you don’t need to expend extra energy. They aim to use this power
to charge cell phones and radios for hikers and soldiers, but that kind of power is enough
to run the small motors we’ll talk about in the next chapter. Another small company,
Lightning Packs (www.lightningpacks.com), uses the weight of a heavy backpack along
with the up and down motion of walking to generate up to 7.4 watts.

Springs and Elastic Energy Storage
6
A spring is an energy storage device, since a spring has the ability to do work. Springs
have many different shapes and sizes. Compression springs are the most common
ones you can find inside mechanical pencils and pens. These will squish a certain
amount when you put a certain force on one end. This force and squish distance tell
us the spring’s stiffness:

Stiffness (k) = Force (F) / Squish Distance (x)

You can sort springs by stiffness on McMaster. We need this stiffness to tell us how
much energy the spring can store. The energy storage depends just on the stiffness
and the amount of distance the spring deforms:

Energy (E) = 1/2 × Stiffness (k) × Distance (x) 2

Although we normally think of springs as coils of wire, the same spring equations
apply to things like diving boards. They are really just long, flat springs, similar to leaf
springs in the suspensions of trucks.

Torsion springs are the kind you find in mousetraps and hair clips that keep them shut.

No matter what kind of spring we’re dealing with, it stores elastic energy. Elastic just
means that as soon as we stop pushing or jumping on it, the spring will return to its
original state. We’ll talk more about different kinds of springs and how to use them in
mechanisms in Chapter 7.