# the physics behind how modern archery bows work

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I’ve spent about 2 hours watching videos on YouTube to understand how bows work but I’m getting more videos that are pitched at comparisons between different modern bows or how to shoot videos.

What I don’t understand is how a compound bow or springed bow is able to make a small person shoot at higher power. In my physics mind, I understand that cams and springs provide leverage. But the force applied to build potential energy is the same direction as the arrow being fired, so how does the arrow have more power?

if I’m using a seesaw analogy, I’m charging potential energy with the end of the seesaw, when I jump off, the far end of the seesaw still has the same distance to the fulcrum

In: 4

Bows? Pretty straight forward. You pull back on the string, turning kenetic energy into potential, because of the elasticity in the arms of the bow. You let go and the string flies forward pushing the arrow.

You’re looking for something more technical?

I think that the compound bow makes use of the leverage your arm produces at various points. Your arm has less leverage the more it’s extended, but gets more leverage as it bends and flexs. So the compound bow makes it easier to pull when your arm is more extended, and shifts the tougher pull to when your arm is more flexed.

Bows? Pretty straight forward. You pull back on the string, turning kenetic energy into potential, because of the elasticity in the arms of the bow. You let go and the string flies forward pushing the arrow.

You’re looking for something more technical?

I think that the compound bow makes use of the leverage your arm produces at various points. Your arm has less leverage the more it’s extended, but gets more leverage as it bends and flexs. So the compound bow makes it easier to pull when your arm is more extended, and shifts the tougher pull to when your arm is more flexed.

Bows? Pretty straight forward. You pull back on the string, turning kenetic energy into potential, because of the elasticity in the arms of the bow. You let go and the string flies forward pushing the arrow.

You’re looking for something more technical?

I think that the compound bow makes use of the leverage your arm produces at various points. Your arm has less leverage the more it’s extended, but gets more leverage as it bends and flexs. So the compound bow makes it easier to pull when your arm is more extended, and shifts the tougher pull to when your arm is more flexed.

Ok, let me try again.

A compound bow, unlike a recurved bow, uses pulleys (or eccentric cams) to wind the bow string on a spring. So, they take less strength, using mechanical advantage to Increas power, versus a recurve bow which doesn’t have mechanical advantage.

So, for example rather than a seesaw analogy, think lever in lifting a weight off the ground. recurve bows are always middle fulcrum, 1 pound down moves the other end 1 pound up. But, compound bows use pulleys and can get different mechanical advantage to pull down with 1 pound but get more leverage in lifting up, in the same way block and tackle can provide greater lifting power

Ok, let me try again.

A compound bow, unlike a recurved bow, uses pulleys (or eccentric cams) to wind the bow string on a spring. So, they take less strength, using mechanical advantage to Increas power, versus a recurve bow which doesn’t have mechanical advantage.

So, for example rather than a seesaw analogy, think lever in lifting a weight off the ground. recurve bows are always middle fulcrum, 1 pound down moves the other end 1 pound up. But, compound bows use pulleys and can get different mechanical advantage to pull down with 1 pound but get more leverage in lifting up, in the same way block and tackle can provide greater lifting power

The cams on a compound bow are not circles, the radius change. That makes it possible to control how large the force it at different part of the draw

A classical log bow will have a higher fore the more you pull back the string, so you need max force when you have the hand just beside your head. That is not the point you can pull with max force. The max force point is also where you need to keep the hand still to aim.

A compound bow can have can initially have a fast increasing force that then remains constant until you reach a full draw where it drops. So you can spread out the work over a longer distance. The peak force can be at a location you can pull harder. It is also lower when you need to hold your hand still and aim. You can have other force curves too like a slow buildup and then a drop.

So a draw vs force curve can look [like this](https://www.brown.edu/Departments/Engineering/Courses/En4/Homework/2014/HW2_data/Force-v-draw.jpg)

It is not the peek forces that determine the energy of an arrow, The work is force x distance so the area below the force-distance graph.

That is not exactly the energy the arrow gets because part of the bow is accelerated too and that energy is wasted. The energy that is wasted to move parts of the bow is less for a compound bow.