Tuesday, February 28, 2012

Physics Week Five

Physics Co-op Lesson Series @ kympossibleblog.blogspot.com

We started our week five class time by taking a brief look at the history of catapult use in warfare, and a discussion of how they work.

Alexander the Great is famous for being a brilliant general and strategist, and he was the first the make great use of catapults as siege weapons.  Later the Romans, and also the Chinese used catapults and trebuchets during war.  The technology reached Europe during the middle ages, and by around 800AD, the catapult was in widespread usage throughout Europe.  Their use continued during the Crusades and in siege warfare in Europe until the 1400s, when gunpowder and cannons became the weapon technology of choice.  The last recorded use of catapults in a siege was the unsuccessful attempt of Cortez's soldiers in the conquest of Mexico in 1521.  In modern times, catapults are used more for entertainment than for warfare - they are featured in movies like Lord of the Rings and provide the "fire power" for the annual Punkin Chunkin in Delaware.

There are two basic types of catapult - those that use tension as the force, and those that use torsion.  Ancient and medieval engineers used a long, flexible arm that wouldn't break when bent backward.  When the arm was released from that position, it flung forward and released the ammunition.  Torsion (strain developed by twisting) was also used as the operating force.  Operators twisted ropes very tightly around the catapult arm and then released them.  When the twisted ropes were released, the catapult's arm would react by springing up and releasing the ammunition.  Both types are a demonstration of Newton's Third Law of Motion - for every action there is an equal and opposite reaction.  They also are a good example of the difference between potential energy and kinetic energy.  Potential energy is being stored, related to an object's position or condition.  A stretched spring or rubber band has potential energy because it is stretched beyond its natural resting position and when released, it will move back, converting the potential energy into kinetic energy - energy that is being used.
mangonel catapult using tension type energy storage device

This diagram and lots of other interesting material about catapults and trebuchets (and lots of other physics subjects too!) comes from the website: Real World Physics Problems.

At last!! It was time to add the rubber bands to our catapults and try them out!

I used a roll of giftwrap paper as the shooting range, so we could mark the distances that ammunition was thrown.  And naturally, the competition was fierce!  It didn't take more than a few seconds for some of the boys to realize that they could amp up their catapult's throwing power by twisting the rubber band, or applying some extra stretch.  This was a good thing, because they were applying what we learned - increasing the amount of tension and/or torsion would also increase the energy generated.  However, there's also a downside to the increased force... it also stresses the catapult frame.  By the time class was over, most of the catapults showed some definite wear and tear, and a couple of them couldn't take the stress.  But we had a lot of fun!

I asked the kids to please promise me that if they used their catapults at home they would not:
  • aim projectiles at their little sisters
  • shoot dad while he was watching TV
  • shoot mom while she was making dinner
  • shoot projectiles towards the family's china cabinet or other delicate breakables
However, I have received reports that at least one student was firing cheese curls and trying to get them in his dad's mouth.  In his defense, I am pretty sure that it was the dad's idea, and I did make mention in class of the fact that crunchy cheetohs make great projectiles.  I just hope the mom didn't have to clean up the resulting orange splats on the wall and floor.

This is the fifth post in a series -
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