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Activity

Catapult from tin can

Summary: 
Make a catapult from a recycled can, to shoot foil balls up to 10m.
Science content (2016 curriculum): 
Physics: Motion and Forces, Newton’s Laws, Gravity (K, 2, 6)
Physics: Energy forms, Conservation of Energy (1, 3, 4, 5)
Physics: Simple and complex Machines (5)
Materials: 
  • plastic spoon
  • popsicle stick, clipped to the length of the spoon handle
  • duct tape, about 20cm (masking tape, pictured, does not hold up to the forces)
  • two mini binder clips
  • mini cup-cake holder, or fashion a bowl from paper to fit in the scoop of the spoon
  • elastic bands of various lengths and thickesses, #32 works well
  • tin can with both ends removed e.g. soup can, or similar. The diameter must not be wider than the handle of the spoon. Alternatively, use a piece of stiff poster tube (see image 5).
  • aluminium foil, to make ammunition balls
  • an open area, where students can fire their ammunition 10m or so
  • measuring tape to record distances, or mark out the area with lines 1m apart
Procedure: 

Catapult background:
This kind of catapult is called a mangonel. It has an arm that swings. (Trebuchet has sling and a counterweight, onager like trebuchet but uses twisted rope).
First catapult (China, 3rd and 4th century BC) Like a giant crossbow. Greeks - 1st century BC. Middle ages (1000 AD) used a huge variety of catapults: mangonel, trebuchet, onager. Last use of catapult in WW1 to project hand grenades.

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 (see image 2).
Use a small loop of masking 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 (see image 3 and 4):
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.

Discuss the forces:
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).

Graph the results:
Ask students to record their distances for each sized ammunition.
Graph the data.

Change the forces:
Ask students how they can make their catapult fire further. (Make the elastic band stronger by doubling it up, or switching for a stronger one.) Graph the new results.

Notes: 

Although I got some reproducibly different distances with different weights of elastic bands, with younger students in Science Club, we did not see a difference between different elastic bands (see photo of graph), probably because they were not pulling the arm back consistently. Try again with older students.

Tongue depressor catapults are always pulled down by the same amount, and different-sized marshmallow pieces go reproducible distances. Can show F=ma

Grades taught: 
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Gr 7
Teacher: 
Ebru Montagano
Ingrid
Taj Badesha
Teaching site: 
Bayview Elementary
Dorothy Lynas Elementary
Eton Arrowsmith Camp
General Gordon Elementary Science Club
JEMZ+ After school science
Kerrisdale Annex Elementary
ProD for Elementary teachers
Activity originally developed and delivered: 

Gordon Elementary Science Club