Bottle accelerometer
The inertia of an object is affected by its density.
What you need
- 2 L PET bottle filled with water
- PET bottle lid
- thumb tack
- 20 cm length of string
- 1 cork small enough to fit through the neck of the bottle
What to do
- Using the thumb tack, attach the cork to one end of the string.
- While holding the opposite end of the string, drop the cork into the bottle. Feed most of the string through the neck of the bottle and secure the end by screwing the bottle lid over the end of the string.
- Turn the bottle upside-down and the cork should float in the water, without touching the sides of the bottle.
- Hold the accelerometer at arms length in front of you and watch the movement of the cork as you walk forward with the bottle. Which way does the floating cork move? Which way does it move when you stop walking?
- Again holding the bottle at arms length in front of you, spin around in a circle. Where does the floating cork appear to move this time?
What's happening?
The floating cork indicates the direction of acceleration. When the bottle moves forward, the contents of the bottle (the floating cork and water) also move forward. The water, having a greater density than the cork, has more inertia. The water forces the cork forward as the water resists the force of the bottle pushing it forward. As you spin around, the floating cork moves towards you, indicating that the direction of acceleration is towards the centre of the circular path.
This can be related to the experience of a passenger sitting in a car when the car suddenly lurches forwards. The passenger's inertia causes them to feel as if they are moving backwards for an instant as they feel the car seat pushing into their back. The passenger’s body forces objects with less inertia, such as the air behind them and the foam car seat, to move out of the way as it resists the car’s forward motion.
