Catapult - torsion

This is called a Torsion catapult. It gets its energy from twisting something. (A twisting force is called torsion).
First documented use by ancient Macedonians (now Greece) in 330 BC, over 2000 years ago. They used twisted rope and tendons. The later Roman 'onager' is an example of a torsion-powered siege engine.

Good for upper Primaries, or students in a smaller space.

Catapult with chopstick frame:
Upper Primaries can build their own. Lower Primaries will need help, or build their own frames ahead of time.
Use an elastic band to bind together two chopsticks at their thin ends.
Measure 7cm from the wide end of each chopstick and make a pencil mark on each.
Open up the chopsticks then hold the popsicle stick between the two chopsticks at the pencil marks. Attach firmly with tape.
Stretch an elastic band between the wide ends of the chopsticks.
Insert the half-popsicle stick through the elastic at the midpoint, then turn it to twist up the elastic.
Without letting go, insert the plastic spoon through the elastic on top of the half-popsicle stick.
The spoon should now be stopped by the whole popsicle stick, so that the elastic does not untwist.
Make sure the spoon is inserted the correct way to use as a bucket for ammunition - if not, turn it while still inside the elastic band.
Hold the chopstick frame upright, pull back the spoon, place a tin foil ball in the spoon, then let go.

Catapult with foam core frame:
Prepare the foam frames in advance - note it takes some time for a class set.
Students build their catapult (with help if necessary): stretch the elastic band over the arms of the foam core. Then use the piece of stick to twist up the rubber band before sliding the spoon in, orienting it so that it acts as the arm of the catapult.

Firing catapults
While staying behind a line, fire balls of tin foil, and record how far they go.
Optionally graph the distances, along with how large the foil ball is.

Discussion of Forces in the catapult:
1. Force from your fingers to wind up the elastic. Energy is now stored in the elastic.
2. Force from the elastic on the spoon to make it move. It pushes the ammo in to the air.
3. The ammo falls because of the force of gravity.
There is a chain of forces.

Ask students how they got the ammo to go further, then they try.
e.g. Pull the spoon back further
Change the angle of attack
Change the size of the ammo.
Wind the elastic up more. It holds more energy. It can move the spoon faster, to project ammo faster.
Double up elastics. Even more energy in the elastic.