Making a catapult – looking at elasticity
Elasticity is the ability of a solid to return to its original shape after being subjected to strain. Most solid materials demonstrate elasticity, up to a point called the elastic limit. When the load is higher than the elastic limit damage to the material occurs. To demonstrate elasticity you can make a catapult.
What you need:
Lollypop sticks or pencils
Lots of elastic bands
A small bottle top
Make a Catapult
This is not an ideal design, so you don’t need to copy us exactly, you just need the basic mechanism.
We first made a square of lolly pop sticks. The corners are secured with a very small elastic band wrapped a couple of times accross the corner both ways. Keep it quite loose initially just to get the basic shape, then go round again and wrap the elastic bands as many times as possible.
The next step was to create two V shaped pieces. Each of these was then secured with each leg of the (inverted) v to the base square. Again, loosely connect, then strengthen once you’re happy with the placement.
Then strengthen the structure with two cross members, one about 1/4 of the way up and one at the top.
Finally, secure the “throwing arm” to the bottom brace somewhere near the centre. We also stuck on a bottle top to contain the ammunition
To be honest, this design evolved just by joining bits of lollypop sticks together rather than through considered design. It has flaws.
1. It is not very even, it has a bit of a “tilt” to it, not all cross pieces are horizontal
2. It has a tendency to flip when loading the arm, it requires holding down the front to pull down the throwing arm. We could add some weights to the base to prevent this.
3. The angle of the shot is a bit steep sometimes discharging the ammunition nearly vertically
4. Lollypop sticks, being flat, are hard to secure in 3 dimensions (especially with springy elsatic). Pencils might work better.
However, it was fun taking the trial and error approach with my 4 year old. At each issue he enjoyed thinking of ways to overcome the problem. For example, to stop it flipping over he suggested we park the car on the front because “that’s heavy isn’t it?”. We opted for the more mundane holding it down with fingers.
We chose to load the catapult with a cannonball taken from a toy pirate ship and a small sweet which was considerably lighter than the toy cannonball. We fired each item three times and marked the furthest distance travelled.
As we expected the sweet travelled the furthest as it was the lightest.
The Science Bit
When you pull down on the arm of the catapult the elastic band holding the lollystick to the crossbar is stretched. Energy is effectively stored in the elastic band. When you let go, the elastic band returns to its original shape. The stored energy has been converted into movement, which makes the arm move fast. When the arm stops moving (it hits the top crossbar) the energy is transferred into the only bit that is free to move (the ammunition). This now flies forward. Heavier objects are harder to move (try pushing a table then a chair). So with the lighter ammunition (the sweet) the arm is able to move faster, so it has more speed when it leaves the holder, this means it can travel further. We used chalk to measure where each object landed.
You could make a bigger catapult with a longer arm and see if that has more power.
Try lots of different objects of different weights and sizes.
Can you make a Catapult?