ingridscience

Show students an image of a local bridge with a visible frame, and ask if they have seen or been over this bridge, or one like it. Tell them that the frame is strong and keeps the bridge rigid, even with the force of all the cars and trucks driving over it.
Show students an image of a frame of a house, and ask if they have seen anything like this on a construction site in their neighborhood. Tell them that behind the walls and under the roof of every building there is a frame like this one that supports the weight of the building and everything in it.
Point out the steel/wooden beams that make up the frame of the bridge/house. Tell students that these beams are fastened together in shapes that are strong, which can distribute and balance the forces on the structures, so that they do not fall down.
Explain to the students that they will be building their own regular polygons, from rods and fasteners, and testing them for strength, to find out which shapes are the sturdiest.

Show the students the rods and fasteners that they will work with (the short plastic strips and the brads), and demonstrate how to fasten the plastic strips together to make the outside of a shape (see photo 1), with three or more sides. Then demonstrate how to test the strength of their shape by laying a square on the table and gently pushing it from one side (see photo 2).

Distribute kits containing eight short plastic strips and eight brads to each student pair. Distribute one worksheet (attached below) to each student.
Students are instructed to make a triangle, a square, then any other simple polygons they like. The shapes should be a simple outline of a shape, with no cross bracing. (Students will be able to build up to an eight-sided octahedron with their kit.) Students should draw each shape in a box on their worksheet, name it if they can, and record the number of sides.
After testing it for strength, they also record on their worksheet how strong and sturdy it is compared to their other shapes (for example using a star system). Polygons that hold their shape well (typically the triangle) are recorded as being the most sturdy (e.g. three stars). Polygons that easily distort and lose their shape (most of the other shapes) are recorded as being the least sturdy (e.g. one star).

Students add the shapes they have built to a class chart, with names and number of sides, until all shapes built are listed for all to see.
Students are asked, pair by pair, which shape they built was the most sturdy, and how sturdy the other shapes were. Record the sturdiness next to the appropriate shape on the class chart. (Some students’ data may be different from others, as sometimes the brads are tight enough to restrict the movement of the plastic strips. However all results are valid, as they are what the students observed, and are added to the data. The outlier data points are not the majority so are not included in the discussion of class results.)
Summarize the class chart: the triangle should be the shape that most often has three stars. Other shapes will vary in their rankings, but should usually rank below the triangle.

Discuss why the triangle is the strongest shape: a triangle will hold its shape even when forces are applied to it from any direction. The shape of a triangle can only be changed by changing the length of its sides, so if its sides stay rigid (do not buckle) the triangle is stable. A force on a triangle is spread around the shape, as compression (pushing forces) and tension (pulling forces), and these forces are balanced. In comparison, a force on a square or other shape with more sides can change the shape by collapsing the corners.

Ask students how they might make the weak shapes stronger. If prompting is needed, ask them which was the strongest shape (triangle), then how this shape might be made within the weak shapes.
Distribute five long plastic strips to each student pair at their desks.
Ask students to make their original shapes, then add cross braces to reinforce the weak shapes and make them strong. Photos 3, 4 and 5 show some examples of the many possible outcomes.
Ask students to retest the reinforced shapes for strength, and look for the shapes (usually triangles) that have been made from the original, larger shape.

Students bring one of their reinforced shapes to a group discussion, and show the triangles that are within it. If there is any weakness in a shape still, ask students how this could be strengthened, and point out the additional triangles that are made.

Look again at the images of the bridge and building. Ask students to find the shapes within the frame, noticing that many or all of the shapes are triangles. Engineers have learned that triangles are strong shapes, so build them into any large structure to keep it rigid and strong.

Show students ideas of mobiles.
Ask them to draw their mobile, probably best with just two sticks for time, and more interesting if one stick hangs from another so the balance point is not in the centre (see first photo).

For materials, thicker twines (e.g. hemp) and heavier objects (e.g. pine cones) allow students to do more of the tying themselves.
For hanging origami, embroidery thread can be stapled to the paper.

When they start to build it, students should work up from the bottom of the mobile.
First hang the materials to each end of the bottom stick, then hot glue in place.
Then attach the thread and move it sideways along the stick until this bottom stick is balanced (ask students to predict where the balance point will be if they have done activities on balance points and relative masses already). Then hot glue in place (with as little hot glue as possible so the balance is not upset).
Then attach the top of this thread to one side of the next stick up (or more centrally if the students prefer a more symmetrical mobile). Add (an)other object(s) to the end(s) of the stick. Tye the hanging thread and move it until the balance point is found, before hot-gluing in place.

Draw/write something on the piece of paper.

Place the drawing behind the card so that it is not visible.
Find the place to hold the mirror so that you can see around the card at your drawing.

Stick the drawing on the underside of a table.
Find the place to hold the mirror so that you can see under the desk at your drawing.

Play around with other ways of using the mirror to see around corners/behind you etc.

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.

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).
The catapult demonstrates Newton's Laws.

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.

Graph the results:
Ask students to record their distances for each sized ammunition. Add meter marks on the floor at the side of the room for students to use.
Graph the data.
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. Although individual students may be able to make the firing distance change, they are likely somewhat consistent with how far back they pull back the catapult arm. With the whole class results on one graph, the variability in how far different students pull the catapult arm back means a graph is rather messy. 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.

Introduction:
I have seen this called a "viking catapult", based on how it is made up of rods lashed together.
The skewers model tree trunks, the elastic band models rope and animal sinews.

Procedure:
Clip the sharp tips off the bamboo skewers. Cut one of the skewers in half.

Refer to the image to see the final shape of the scaffold, and follow this order of construction if you like:
Use an elastic band to secure three skewers together at one end.
Use another elastic band to secure the other end of one of these skewers to two more skewers (at their end).
Take one free skewer end from each bundle, and bind them together - this should make a triangle.
Take the last free skewer end from each bundle, and bind them together - this should make a second triangle. The two triangles you just made have one skewer in common.
Hold the two triangles apart from each other and brace them open with the two short skewers, using elastic bands at each joint, as in the photo.

To make the ammunition pouch:
Loop an elastic band through each of the holes.
Two of these elastic bands can hook over the upright struts of the catapult.
The third elastic band should be secured to the bottom skewer with one or two half hitches.

Make ammunition from balls of aluminium foil.
Hold the ammo in the pouch as shown in the second photo, then release. It should go several metres.

Show students how to read their thermometer, making sure not to cover their hand over the bulb.
Students make temperature measurements at different places in the pond. They record each temperature measurement on correct location on the map.

Discussion: was there a change in the pond temperature?
We found the shady spots were a little lower in temperature.