Tuesday, March 20, 2012

Physics Week Eight

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

We had some successes and some semi-failures for our Physics class last week.  The jury is still out on our Viking catapult.  I built this catapult at home and brought it to class for the students to try.  The instructions are in the book The Art of the Catapult by William Gurstelle.

The hardest part for me was figuring out the clove hitch knot to get each of the lashed joints started.  I am knot-challenged, apparently.  I had to find a "how-to" on youtube and even then I struggled each time.  LOL  But I've learned a new survival skill, so if I ever need to lash together a sturdy framework for a lean-to...

The finished frame looked like this:
That's when we ran into some difficulty.  The instructions recommend using an elastic rubber tubing usually sold in medical supply stores - the type of band the lab technician uses on your arm when you need to give blood.  Unfortunately for me, neither of the local pharmacies that sell these kinds of supplies had that tubing available.  (One of the ladies told me that nobody buys it any more except people who are making slingshots! LOL)  With time running out, I had to make do with wide elastic, and sadly, that just didn't provide enough power for effective flinging.

We got the general idea though (the above pictures are of Kennady testing it in our yard), and I told the students that I would bring the Viking Catapult back when I found something better - bungee cords maybe?

Background on the Viking Catapult - these were tension-powered catapults, using the energy stored in the fibers of a resilient material to provide the necessary force.  Written accounts from AD500-1000 tell of frequent attacks on English and French towns by Viking raiders, and indicate that these types of siege weapons were used.  One record by the French monk Odo in the late ninth century states that the Vikings used catapults in their attack on Paris.  No drawings or examples exist, but based on descriptions and what is known about how Vikings built their other siege engines, wagons and ships, this is a speculative model.  They built using round poles (sorry, mine are square - it's what we had available!), broad timbers, and sturdy roped connections.

Now on to the real point of the lesson!  We talked a bit about momentum.  In straight-line motion, momentum is a function of mass and velocity.  But objects moving in a circle have angular momentum, which is a function of the radius of the circle in addition to mass and velocity. (We do not take the time to discuss the mathematical equations in class - just the concepts.)  Centripetal force is the force that keeps a rotating object moving around in a circle.  It is pushing or pulling the object toward the center of the circle.  When you swing a bucket of water, the water stays in the bucket because it exerts an outward force (centrifugal force) that is equal to the inward centripetal force exerted by your arm and the bucket handle, which keeps the bucket moving in the circle.

Centrifugal force is used in machines such as the centrifuge, which is a device that spins liquids - two common examples are the centrifuges used to separate milk and cream in a dairy, and those used to separate heavy red blood cells from plasma in a medical lab.  Centrifugal force is also used in many seed and fertilizer spreaders, roller coasters, and of course on a merry-go-round.

We made some very simple hand slings, with just a scrap of material and two lengths of cotton cord.  (again, following instructions in The Art of the Catapult.)  It only took a few minutes to tie the cord onto the ends of the strip of fabric and then we went outside to try them out.  I had the kids bring a munitions supply of water balloons and other small (relatively safe) projectiles, and we spent most of our class time getting everyone wet and seeing how far we could launch the balloons using the hand slings and the modified staff sling I'd brought.

Our hand sling was based on the ancient Balearic style.  Hand slings allow you to throw stuff further than you could with just your arm, because the sling effectively increases the length of your arm and the velocity of the object at release (a function of angular momentum, just like the sling action of our trebuchets).  Slings have been used as effective weapons since ancient times.  Alexander the Great employed mercenaries from Rhodes, known for their skill and accuracy with a sling. The city of Thrace in Persia, and the Balearic Islands in the western Mediterranean, were also known for the skill of their 'slingers'.  Good 'slingers' were just as important to the armies as archers, because they could hit moving targets from a long distance with deadly force and accuracy.

By fastening the cord to a staff (I used a broken off hockey stick!), we fashioned something similar to a Thracian Staff Sling.  These were used in battles by Thracian soldiers called Peltasts (which means slingers).  The Peltasts were lightly armed, fast-moving foot soldiers.  An advantage of a staff sling is that it is much easier to aim.  The disadvantage is that the velocity of the projectile is considerably less than one thrown with a hand sling.  Most of the kids that tried the staff sling preferred it - it really was easier to aim and get a good long throw.

And as a bonus, my kids have a new toy.  The hockey-stick/staff-sling has already provided lots of entertainment at home!

[David], with his sling in his hand, approached the Philistine... Reaching into his bag and taking out a stone, he slung it and struck the Philistine on the forehead... So David triumphed over the Philistine with a sling and a stone. ~ I Samuel 17

This is the eighth post in a series -

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Jennifer said...

What a fun way to teach and to learn physics.

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