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Catapult - torsion

Build a catapult, experiment with how far it can project, and discuss the forces acting as it works, including gravity.
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)
Science topic (2005 curriculum connection): 
Physical Science: Force and Motion (grade 1)
Physical Science: Forces and Simple Machines (grade 5)

For each catapult:

  • foam core/very stiff cardboard in a square U shape - see notes on cardboard
  • elastic band, #64 works well and fires 4-5m, #32 works OK but has less power
  • plastic spoon
  • small section of popsicle stick (cut one in thirds)
  • piece of aluminum foil
  • firing line: either lines already in gym or masking tape
  • metre markers (see attached file) to post on walls/floor

We will build a catapult that works with the forces we have been talking about.
Students each build a simple catapult. (Stretch the elastic band over the arms of the foam core. Use the piece of stick to twist up the rubber band, then slide the spoon in, orienting it so that it acts as the arm of the catapult).
Once everyone is done, tell them the task: while staying behind a line, fire balls of tin foil, and record how far they go.
After 5 mins, ask what they did to make the balls go the furthest. (Direction of the force, strength of the force by pulling the spoon back more or less, weight/size of the object being fired, strength of the force by twisting the rubber band more or less).
Then more time to try out other students’ suggestions. Ask them to think about the forces on the paper ball.
Discussion on the forces involved in each stage of the catapult firing (force from hand to pull back the spoon, force in the elastic band (potential energy), force as the elastic band unwinds and moves the spoon, force of the spoon on the foil ball, gravity pulling the foil ball to the ground). There is a chain of forces.

Students can measure and record on a graph how far their ammuntion goes.

More details on catapult forces:
It is called a torsion catapult. (In the twisted band - a twising force is called torsion).

Attached documents: 

Cardboard from a thick box collapses at the bottom of the U after a couple of tries. Stick with foam core or test your cardboard well first.
The U shape is a lot of prep for a whole class, but the simple build is good for younger students.
Try other catapult designs from
Mme Romy's class had to figure out how to put the catapult together, before using it to fire.

Grades tested: 
Gr K
Gr 1
Gr 2
Gr 4
Gr 5
Gr 6
Anke Barabulea
Annie Lee
Donna Greening
Kelli Madsen
Romy Cooper
Teaching site: 
David Lloyd George Elementary
General Gordon Elementary
Kerrisdale Annex Elementary
ProD for Elementary teachers
Activity originally developed and delivered: 

Gordon Elementary