The Science of the Helium Balloon in a Car

All objects have inertia, which means they try to stay in one place unless a force acts upon them. This is why when a car accelerates forward, you “feel” as if you are being pushed backward into your seat; in reality, your body is simply trying to remain in the spot where the car was parked a moment ago.

This inertia also affects the air inside the car. As the vehicle moves forward, the air molecules inside are left behind momentarily, causing the air density and air pressure to increase at the back of the car.

Normally, you would expect the helium balloon to drift to the back of the car along with the air. However, because helium is significantly less dense than the surrounding air, it is subject to buoyancy. Just as a balloon floats “up” away from the Earth because air is denser at the ground, the balloon “floats” forward away from the high-pressure air at the back of the car. This is a practical example of Einstein’s Equivalence Principle, where acceleration acts exactly like a gravitational field!

Variables to test

Find out more on variables here.

• Density Comparison
  Try a balloon filled with normal air instead. Does it move forward like the helium balloon, or does its higher mass cause it to swing toward the back of the car?
• Acceleration Force
  What happens if the car turns a corner instead of driving straight? Does the balloon move toward the inside or the outside of the turn?
• Incline
  If you park the car on a steep hill, does the balloon stay in the same relative position, or does the change in the “upward” direction affect its resting spot?