For those of you that have kids, today's thought may prove to be a fun little DIY science experiment. For those of you with young adults, maybe you can get a free chore or dinner out of it by winning a 'bar bet'.
DIY: If you have a standard helium filled balloon in a stationary car, and hit the gas, what happens to the balloon?
TOOLS:
- A car and legal driver
- A balloon filled with helium
PROCEDURE:
Lets first start by thinking what happens to a person. Assuming no other parlor tricks, or interpretation of the phrase "hit the gas", when you accelerate quickly in a forward gear, the car is going to accelerate in a forward direction. Detached objects in the car (people, water bottles, etc...) are going to be affected by inertia and resist the forward pull of the car. Hence you will be thrown back into your seat until the seat, which is attached to the car, accelerates you so that you are traveling at the same speed as the vehicle. Similarly, when you brake, that same force will tend to keep you moving in a forward motion as the car slows down or through the window in the case of a sudden stop (PSA for Seatbelt Use).
The astute among you likely already know where I'm going with this and will not be surprised at all when I tell you that when a helium balloon is in an accelerating car, it moves to the front and not the back. Now I'm sure there is a combination of "What?", "Huh?", and "I don't believe you" as you read that last statement, so let me go into a bit more detail as to why.
Think about a jar of water with a bubble in it (or more conventionally an old fashioned level). The bubble, being less dense than the water, in a place with gravity is always going to float to the top. This is a good analogy for what is happening in the car itself. By looking at the molar masses of helium and the air we breathe, we can calculate that the air is approximately 7x more dense than the helium, and as such we have set up our own little jar (the car), the water (the air in the car), and air bubble (the helium balloon). When the car accelerates, the air in the car is pushed towards the back of the vehicle and the balloon, being less dense, is moved forward.
Next time you are driving and get upset because your big mess of balloons are blocking the views in the front seat, just remember that the best solution is to drive in reverse. That, and charge the back seat passengers for using all the good air when you are accelerating.