Horizontal Floating? HUH?

You are driving in your souped up Hyundai to your girl/boy friend's house with helium balloons in the back seat as a gift for his/her acceptance to Harvard Law School. A small furry creature darts in front of your car and you instinctively slam on the breaks - I certainly wouldn't, but YOU do. Explain the motion of you, the helium balloons in the back seat, and the small fury creature after he sees you approaching him at 90 MPH.

Most kids, and teachers, will correctly answer that you, as a mass in motion, will feel a fictitious forward force that is really just your own inertia tending to keep its motion in a straight line forward as the car slows down first - Uncle Newton's 1st Law. Most of us will answer the same for the balloons. However, the balloons will NOT MOVE FORWARD, very contrary to Physics Intuition. They will actually move further to the back of the vehicle! How come and why for?

It's what I call "horizontal floating". The same principle that causes warm air to "rise" - that is, the cooler air is denser and therefore sinks in the atmosphere simply pushing the warm air out of the way. The air in the car, of density about 1.3 kg/m3, is MUCH more dense than the He in the balloon, at density about 0.18 kg/m3, and will thus possess much more inertia. When the car slows down quickly, the air in the car still moves forward under its own inertial properties. The He balloon is simply pushed out of the way from the air moving forward and ends up in the back of the car.

How to demonstrate this amazing effect?

• Mini-Fieldtrip to the parking lot. I used to have me videotaped as I drove by with a balloon in the backseat. I now borrow all the digital cameras the school can muster - many of the kids have them now, too - and line the kids up at about 10 meter intervals. As I drive by this line of kids, I slam on my breaks when I get to the 2nd kid. Each kid is responsible for taking an action shot of the car and contents as it passes their position. We then rush up to the Fizzix Lab and download the digital pictures into the one computer I have and watch frame to frame the relative motion of the balloon AND ME. I move forward in my "resting" position. The balloon actually moves to the rear of the car.

Many times, several trials are necessary to get all the shots you need. However, the kids LOVE it and they get immediate scientific feedback from their own actions. Try it, you'll LOVE it!

Candle in Jar: A slightly more conventional method is to have a lit candle in a large sealed jar. A large mayonnaise or pickle jar is good. Climb up on a desk, light the candle, tighten the lid, and drop said jar onto a pile of cushions. As it falls, kids observe the flame goes OUT after actually being pushed DOWN. Most will hypothesize the flame will stretch upward as it wants to be left in its original position - quite a bit like a cartoon character leaves part of his body behind as he falls. Why? The air in the jar, being more dense, will "move" to the top of the jar as the jar falls - just like moving forward when the car stops. Thus, the air actually "blows out" the candle.

Ziploc and Ping-Pong balls; The same thing can be accomplished using a large Ziploc bag 3/4 full of water with a ping pong ball in it. Hold the bag so the ball is pinched by your fingers at the bottom of the bag. Let it fall. The ball stays at the bottom instead of rising to the top as the kids would expect. Why? Same reason as all the others; the water's inertia forces it to "move" to the top of the bag pushing the ball out of the way so it looks like it sinks.