To make the catapult arm:
Tape the short popsicle stick to the spoon handle, to reinforce it.
Lay the end of the spoon handle over a silver handle of the binder clip, and temporarily open up the binder clip, so you can tape them together tightly with duct tape.
Use a small loop of tape to secure the min cup cake holder in the scoop of the spoon - this makes a deeper bucket for the ammunition.
To attach the catapult arm:
Clip the spoon handle with its binder clip to one lip of the tin can, so that the catapult arm can swing up to the other side of the can.
Attach the second binder clip to the other side and the other end of the can.
Loop an elastic band over the scoop of the spoon, across the can, and behind the inside handle of the second binder clip. The catapult arm should be pulled up against the rim of the can.
One of each of the binder clip handles can be flipped back, to make the attachment more secure.
To fire the catapult:
Make ammunition from balls of aluminium foil.
Load the ammo in the bucket, while tilting the catapult backwards, so the ammo doesn't fall out.
Pull back the arm of the catapult by the reinforced handle, while moving the can back to its horizontal position.
Let the arm go. The ammunition should fly several metres.
(In the classroom students can fire the catapult without any ammunition, to understand how it works.)
Discuss the forces and energy transfers:
When you pull the arm back, energy is stored in the elastic band. As the arm is released, the elastic band contracts again, exerting a force on the catapult arm and pulling it forward again. The bucket of the catapult exerts a force on the ammunition, pushing it forward with it. When the arm hits the can, the ammunition has no force to stop it from moving, so it continues to project forward. Gravity pulls the ammo downwards as it moves, so it makes an arc across the room.
The catapult is a class 3 lever, with the effort (the elastic band pulling on the catapult arm) between the fulcrum (the binder clip hinge) and the load (the bucket). The bucket moves further than the spoon handle, but experiences less force at one time (though has plenty of force to move the ammo forward).
Discuss Newton's Laws in the catapult:
1st Law: Focus on the ammo - catapult arm makes it move. Gravity makes it fall. Air slows it down. The ground pushing up makes it stop.
2nd Law: F=ma. A greater force (stronger band/pulled back more) will make it go further. A larger mass will go less far.
3rd Law: At each step there is a force pushing back on the ammo, even when it is still. When one force is greater than the other, the ammo moves or slows down.
Take the students outside to fire their catapults.
Scrape out lines in a gravel field or chalk up concrete, to provide the students with metre marks from a firing line. Students can stand on the line and measure how far their ammo flies each time.
Change the forces:
Ask students how they can make their catapult fire further (also helps students that are having trouble). Ideas: make the elastic band stronger by doubling it up, or switch for a stronger elastic band. I use three elastic sizes, #32 to start with, then add in #16 and #64. The sizes do not matter, but these bands do work.
Graph the results:
Ask students to record their distances for each sized ammunition.
Graph the data, with students initialling their places on the graph (see photo).
Note that I have not been able to make any consistent pattern from graphing whole class distances, either from different sized balls or different elastic bands. Neither is there no clear correlation between the distances fired and different elastics when used by the same student with only a few data points.
It is still OK to graph and see that there is no distinct pattern - opportunity to ask what other variables there might be, and how to test for these. Also an opportunity to explain that sometimes we need more data to see if there are any patterns. I want to try students each graphing their own data - and collecting around 10 data points each. (I think they will also more naturally want to collect more data to find a pattern.)
