A rubber-band launcher
Stretch a rubber band and fire a paper pellet. Change the band, how far you stretch it and the pellet's mass, and see the energy stored in the band, the launch speed and how far the pellet flies.
Core concept
A stretched band stores elastic potential energy E = ½kx²; on release it becomes kinetic energy ½mv², so v = √(2E ÷ m), and at 45° the pellet flies about v² ÷ g: twice the stretch gives four times the energy and twice the speed
Topic: Science around us
When you stretch a rubber band you do work on it, and it stores that work as elastic potential energy: What is needed to make things move, heat up, light up or grow. All key words. For a band of stiffness k (newtons per metre) stretched x metres, the energy is E = ½kx². The x is squared, so stretching twice as far stores four times the energy. When you let go, the band gives its energy to the pellet as kinetic energy, ½mv², so the launch speed: How far something travels in a certain time, like kilometres per hour. All key words is v = √(2E ÷ m). A lighter pellet gets a higher speed from the same energy. Fired at 45°, the pellet flies a distance of about v² ÷ g, where g = 9.8 m/s², if we forget air resistance: How much something slows down the flow of electricity, measured in ohms (Ω). All key words.
What you will learn
- Work out the elastic energy stored in a stretched band.
- Use energy conservation to find the launch speed.
- Explain how the stretch and the mass change the range.
Before you start
- Know that kinetic energy is ½mv².
- Know how to change cm to m and g to kg.
Materials
- Rubber bands of different thickness
- Paper pellets and a ruler
Equipment
- A measuring tape
Procedure
- Record the thick band stretched 10 cm with a 20 g pellet.
- Stretch it 20 cm and record.
- Change to a 50 g pellet, keep 20 cm, and record.
How this simulation is simplified
- Band stiffness: thin 50 N/m, thick 150 N/m, two bands together 300 N/m; the band obeys Hooke's law.
- All the stored energy goes into the pellet; it is fired at 45° and air resistance is ignored (g = 9.8 m/s²).