Imagine hanging a pendulum on the inside of your car and accelerating forward. Which direction do you think the pendulum will move? Based on our common life experiences, we would expect the pendulum to move backwards. Our bodies experience this whenever we ride in a moving vehicle. Imagine the last time you knew your roller coaster ride was about to begin. As the coaster train accelerates forward, you lurch backwards and feel your back pressed harder against the seat.
Now imagine tying down a helium-filled balloon inside your car and accelerating forward. Which direction do you think the balloon will move? As Destin from the popular YouTube channel Smarter Every Day demonstrates, the balloon actually moves forward when the car accelerates forward. This is the opposite motion that happens to the pendulum and our bodies. Why does this happen?
We know from Newton’s first law that an object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted by an external force. The pendulum, car passenger, and helium-filled balloon all have inertia and thus want to “stay at rest” in their initial position even as the car moves forward. But why does it seem like the helium-filled balloon is pushed forward instead kept backwards when the car accelerates? The answer can be visualized with an analogy.
Imagine the whole vehicle to be filled with water instead of air. An air bubble rests on top of the water, just like a helium balloon resting on top of air. This is because air is less dense than water, and helium is less dense than air. When the car moves forward, all of the water sloshes back, leaving the air bubble to occupy the space in the front. Taking this analogy further, we can imagine that as the car moves forward, the air “sloshes” back, leaving the helium balloon to move forward to occupy the front of the car. In other words, in the short amount of the time the vehicle accelerates forward, the passengers in the back of car temporarily have more air to breathe than the driver! Although this effect is temporary (it only lasts until the car reaches a new constant velocity), it makes us remember the substance and mass that is air.
Less obviously, this experiment can help us realize is how much we take air for granted. Just like how fish swim in water, we spend time in, walk around in, and live in a world surrounded by air. We don’t notice it until it’s polluted and no longer pleasant to breathe in. So the next time you step outside and enjoy the coming spring weather, take a moment to appreciate the non-noticeability of good air!