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Bottle Accelerometer

A upside down plastic drink bottle filled with water, and a cork floating to the top with a string falling down out of the cork. The bottle is held by one hand at the bottom.

Full steam ahead and watch which way the cork goes. 

What you need

  • 2 L PET bottle and lid
  • water
  • thumb tack
  • 20 cm length of string
  • 1 cork small enough to fit through the neck of the bottle

What to do

  1. Fill the PET bottle with water.
  2. Using the thumb tack, attach the cork to one end of the string.
  3. 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 loose end of the string by screwing the bottle lid over the loose end.
  4. Turn the bottle upside-down and the cork should float in the water, without touching the sides of the bottle.
  5. Hold the accelerometer at an arm's length in front of you and watch the movement of the cork as you:
    • walk forward,
    • walk backwards and
    • spin around in a circle.

What's happening?

The cork moves in the direction of acceleration. As you accelerate forwards, the water moves to the back of the bottle, pushing the cork forwards. As you accelerate backwards, water moves to the front of the bottle, pushing the cork backwards.

This experiment works because of the different densities of the cork and the water. Density is a measure of how much mass is in a given volume. Density is important when discussing moving objects because the density of a material affects how it behaves under inertia. Inertia is the tendency for something to stay stationary or keep moving at the same speed. Objects with a higher density are more likely to keep moving at the same speed and resist having their speed changed.

As the bottle accelerates forwards, both the water and the cork are affected by inertia and try to stay in their original position. However, the water, having a greater density than the cork, has more inertia. This means the water has a greater tendency to move to the back of the bottle and stay in its original position than the cork does. The less dense cork is therefore pushed forward in the bottle to make way for the water at the back of the bottle. As you spin around, the floating cork moves towards you, indicating that the direction of acceleration is towards the centre of the circular path. Thus the cork will always point in the direction of the acceleration.

The effects of inertia 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.

Did you know?

Magicians use inertia to pull a tablecloth out from under a set table. When all the objects on the table are sitting still, they will have a tendency to stay sitting still. When the magician pulls the tablecloth out very quickly, the inertia of the plates and dishes keeps them from flying off the table. The faster the magician pulls the cloth, the better this works. It also helps if the cloth is made of something slippery, like silk, to reduce the pulling force of the cloth on the plates. You can try this at home yourself. It does take a bit of practice though, so make sure you don't use your good china plates.