## Which way does a bubble go when the water around it accelerates?

This experiment provides an example how inertia can play tricks on you using a substance that has an inertia that everyone is well aware of - water.

All you will need for this experiment is an empty soda bottle (the 2 liter ones work very well) with its label peeled off and the bottles screw cap.

What to do:

1) Fill the bottle almost to the top, and then seal it with the cap. 2) To check whether the bottle is filled tot he right level, tip it on its side and let it sit flat. There needs to be a small (maybe one inch across bubble of air ont he top side of the bottle. If the bottle is nice and regular, the bubble will probably be floating near the middle of the top side.

3) Now, before you go on, try to guess what the bubble will do as you move the bottle back and forth along its longest direction (or from cap to base if you like). Also, think about what you feel happens when you're in a car that is accelerating of braking. If you've already tried "A Candle in a Jar" you'll probably know what is going to happen!

4) Gently move the bottle, still held on its side back and forth along the line between the cap and the base.

5) Watch the bubble as the bottle moves. What direction is the bubble moving in (relative to the bottle) when the bottle is speeding up or slowing down?

## What is happening? Why is the bubble going the "wrong" direction?

When you ride in a car that is accelerating, you feel a "force" that pushes you in the opposite direction. When the car speeds up, you're pushed backwards into your seat. When the car turns left, you're pushed to the right. When the car brakes, you're pushed forwards. These "forces" are not true forces - nothing is actually pushing you forward when the car brakes suddenly. Instead, these "pseudoforces" are due to your inertia and its tendency to travel in straight lines at constant speeds (Newton's First Law).

However, the bubble moves differently. When the bottle accelerates to the left, rather than trailing to the right the bubble moves even faster to the left! Then, when the jar is moving at a constant velocity the bubble is stationary relative tot he bottle. Finally, as the bottle decelerates back to stationary, the bubble trails to the right!

When everything is stationary, the bubble floats, stationary at the top of the bottle. This is because themuch denser water around the air bubble "sinks" to the bottom of the bottle, displacing the bubble - the air has to sit at the top of the bottle.

When the bottle is accelerating, the inertia of the water (and the air) causes it all to be pushed to the back of the bottle (just like the way you are pressed back into your seat in an accelerating car). Because the water is heavier than the air, it accelerates slower and is pressed back harder against the back of the bottle, pushing the air slightly forward as well as upwards. Thus, the bubble floats forwards. Similarly, when the jar comes to a stop, the water goes to the front, pushing the air to the back.