What it shows
Pulling the arm down stores energy in the bent stick and rubber bands, and letting go turns it into the motion of the pom-pom. More bend means more stored energy and a longer flight, and the launch angle changes the distance too.
The science
When you bend the stick and stretch the rubber bands you do work on them, and that work is stored as elastic potential energy. When you release the arm, the stick springs back and gives this energy to the pom-pom as kinetic energy, so it flies off. The more you bend the arm, the more energy is stored and the faster and further the pom-pom goes. Once in the air, gravity pulls it down in a curved path, and a launch angle of about 45 degrees gives the longest range.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- About ten ice cream sticks (free from a kulfi stall, washed)
- Six to eight rubber bands
- A plastic spoon or a bottle cap as the cup
- Pom-poms, paper balls or balls of foil to launch
- A measuring tape and chalk
Safety
For this project: Launch only soft things like paper balls, never stones or hard objects, and never aim at a person or an animal.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Stack eight sticks together and wrap a rubber band tightly around each end to make a thick block.
Take two more sticks, place them one on top of the other and band them together at one end only.
Push the thick block between the two sticks near the banded end, so one stick is pushed up like an arm.
Band the crossing point so the block stays put, and tape the spoon or cap to the end of the raised arm.
Place a pom-pom in the cup, press the arm down a little and let go; mark where the pom-pom lands.
Repeat with the arm pressed halfway and then all the way down, measuring each distance and taking three tries each.
Tilt the whole catapult by putting a book under the back and see whether the pom-pom flies further or shorter.
Write a table of how far the arm was pressed against the average distance and draw a bar chart.
What you should see
A working catapult that throws a pom-pom a metre or more. The further the arm is pressed down, the further the pom-pom flies, and tilting the launch changes the range.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Where is the energy stored before launch?
In the bent stick and the stretched rubber bands, as elastic potential energy.
2What kind of energy does the pom-pom have in the air?
Kinetic energy, the energy of movement.
3Why does the pom-pom come down in a curve?
Gravity pulls it downwards all the time it is flying forwards.
4Why use more sticks in the block?
A taller block bends the arm more, storing more energy and giving a longer throw.
Take it further
Launch a pom-pom and a heavier foil ball with the same bend and find out which goes further and why.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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