Eli5 How does stored energy work? Especially when it comes to differing materials.



I’m reading an explanation of why composite bows were so much more powerful than the wooden bows before them because the animal horn and sinew “stored far more energy.”

I think this is that lesson in physics that I never fully understood and only passed because I memorized basic terms.

In: Physics

Energy can be stored by materials in many ways. In terms of a crossbow, energy is stored by the physical change in shape of the object. As you push on it, it bends, and so a force is applied over a distance. As you release it, it unbends itself and applies a lot of force to do so, returning that energy.

To store more energy, you need to either bend it farther or make it push back harder. By thickening an object, you trade off these two aspects. Wood also doesn’t return anywhere near 100% off the energy you put in.

The material properties we are looking for here are how elastic yet stiff something is. Somethings are very stiff; when they are bent, they bounce back quickly and with a lot of force. Other things bounce back slowly and softly.

Think of rubber bands. Stretching one thin rubber band out 6 inches takes very little effort, when you release, that small amount of energy is released. When you stretch a thick rubber band 6 inches, it is harder to stretch, and it releases all that extra energy when released.

When you shoot yourself in the eye with said rubber bands, you’ll understand how the different rubber bands transfer that stored energy.

Another reason is the recurve shape of the bow. The recurve shape is more efficient in energy transfer.

It basically comes down to stress. On a longbow the material isn’t under as much stress as on a recurve bow when it is strung. The extra curve increases the tension on the bowstring.

It might help to understand WHY a bow needs to “store energy”. (Note that in physics, “energy” and “work” are the same thing).

Shooting an arrow with a bow uses work done by your arms, so why use the bow? Why not just throw the arrow?

It takes about 0.5 seconds to draw a bow, but only 0.01s to shoot. The bow stores up 0.5s worth of output from your arm and releases it all at once (relatively speaking).

The way the bow stores energy is to use work done by your arms to bend wood into a configuration it does not ‘want’ to be in. The wood ‘wants’ to go back to its original shape, and the bow is made so that in order to get back to its original shape, the wood has to pull the bowstring, which pushes the arrow forward.

All forms of energy storage involve putting something into a state it doesn’t normally “want” to be in, which always takes work to do (which is how the work/energy gets stored). E.g. you can store energy in a wheel by spinning the wheel really fast with your hands. You can store energy in water by carrying it up higher off the ground. You can store energy in a spring by stretching/bending it, and that’s what a bow is: a spring.

What makes the wood used in a bow good at storing energy this way is A: the fact that it gives back a decent amount of the energy you put in (nowhere near 100%, but much better than, say, corn stalks, which would make a really terrible spring), and B: the fact that it can release the energy it stores by being a spring fairly quickly.