This last week in physics we were given 5 days to design, build, research, and learn about one of the world’s greatest ancient inventions. My group of 3 decided to chose the crossbow.
The history:
The crossbow was invented independently in ancient China during the Zhou Dynasty in 200 B.C. Then later was developed in Europe/China towards the 5th and 6th centuries. A chinese text from around the time the first crossbows were reported gives credit to the Chinese emporor, Mr. Ch’in of Ch’u for the invention of the crossbow. Ironically, the crossbow (a weapon) was originally invented to try and create peace. 
Mr. Ch’in of Ch’u had the theory that due to the crossbow being so advanced for its time that when other people saw it, they would stop fighting and follow those who possessed it. In fact, in fear that other countries would find out about the crossbow and would try to replicate China’s newest technological advancement, a law was passed restricting all crossbows from leaving the country. Short term, the crossbow was a strong military advancement that made China a powerful force from both near and far ranges. Having smaller crossbows being manned by a single person for closer ranges and larger crossbows (varying in designs such as the ballista, a catapult like object) requiring mulitple people for longer ranges.
An ancient crossbow from 5th century China.
As time went on, upgrades and improvements kept happening to the crossbow, becoming sleeker, more compact, easier to carry. Eventually even implimenting metal into the design instead of the original straight wood design, which meant a stronger, faster and more deadly weapon. However, eventually with the invention of guns. Crossbows today are much less useful and rarely used as weapon of choice in war, being mainly used as a solely hunting weapon and in archery dedicated ranges.
The physics involved in a crossbow:
Before loading the projectile into the crossbow, you pull back the rubber band, which creates potential elastic energy. The rubber band then stores the potential elastic energy right up until you let go, when the potential elastic energy becomes kinetic energy, which is what launches the projectile forwards. When releasing the band, there is both momentum being added to the projectile and the rubber band is applying force to the object and pushing it forward. The amount of momemtum being applied to the object is dependant on two things, the distance back the band is being pulled and the draw weight on the crossbow. However, due to our crossbow having a trigger, the distance back and weight each time should be about the same. The crossbow also is effected by Hooke’s law, which states that the extension of a spring/elastic is directly proportional to the load applied on it. Hooke’s law can also be represented by 
The crossbow also takes part in projectile motion, meaning the projectile (if shot straight) will start coming our straight from the height of the crossbow, then very quickly as gravity starts to take effect on the projectile it will start its descent back towarss the earth.
The design and building process:
The original rough sketch of our day 1 crossbow
Day 1: We spent the first day deciding what ancient invention we wanted to build and research, then once we had finally decided as a group we started talking about what we wanted ours to look like due to their being so many crossbow variations. We decided that it would be in our best interest to use a pullback rubber band, instead of actual string or a spring, and to keep it plainly out of wood as much as we could. Having the only exception being the nails to keep the rubber bands in place.
Day 2: We started the day looking online at various popular crossbow designs, however we realized whilst doing this that most of the crossbows required both special equipement and parts to make. Both being things we didn’t have, so we went back to our original day 1 sketch to see how we could improve that design to make it the best possible within our capabilities. 
We decided we would do our best ro add a trigger that would hold the rubber band with tension until released, we also made a list of supplies that we were going to need for day 3 and who was able to bring what in.
Day 3: We brought in our supplies, which included a bunch of pieces of wood, rubber bands, and nails. However, we ran into a problem when we realized we didn’t have tools such as a saw to cut the wood correctly. 
Being already midway through the challenge and not wanting to lose any more of our precious time, we drew where we were gonna cut the wood for the trigger and how long we wanted the crossbow in class, then cut the wood from home.
Day 4: We spent day 4 doing the research part of the project, learning the history and the physics behind the crossbow. We also spent some of the day doing finishing touches on the crossbow itself and made sure it works. 
Something that worked really well for us as a group was that the rubber band had an ideal stretch for where we had originally wanted to put the trigger on the crossbow, when pulled back it has a good amount of resistance but it’s not difficult to do at the same time.
Day 5: On the last day of the challenge as a group we compared our research from day 4 to make sure everyone had everything incase they couldn’t find something or we got completely different answers. We also started the individual part of the project, our blog posts to be as efficient as possible in class and have less work to do at home.
Sources:
https://www.wired.com/2012/08/do-rubber-bands-act-like-springs/
https://science.howstuffworks.com/crossbow.htm
http://www.bestcrossbowsource.com/history-of-the-crossbow-origins-and-evolution/
https://www.thoughtco.com/the-invention-of-the-crossbow-195263
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