A Bus Full of Children or Gravity? - Elannah De La O

Back home in Sonoma County, California, everyone has heard of the myth surrounding Gravity Hill. The tale is that if you encounter a strip of road on Sonoma Mountain, place your car in neutral, and the souls of a bus full of children will push your car to the top. Although I have never gone up this hill myself, my aunt and her children have and swear in having children's handprints on the back of their car afterwards. In thinking of this hill in conversation with my sister, it made me wonder if there was a possibility to explain the physics behind this.

A video of Sonoma Mountain's Gravity Hill taken for SFGate

How does one see a hill inclining and roll forward on neutral? In this case, it is an optical illusion. As explained by Elizabeth Borneman, a small strip of road such as Gravity Hill is an optical illusion by obscuring someone's horizon line partially or fully, which will affect how they perceive what is up and down. Sarah Kirker draws for SFGate the image below:

Typically, we are the top of the image. Our line of sight goes in front of us where we can see and perceive what is up and what is down. If we are perpendicular to the horizon line and know so, we can perceive an upcoming hill that goes upwards. If we are perpendicular to a downhill slope without being able to see a horizon line as featured on the bottom, we will see a straight or less steep downhill slope as an incline instead. For some roads, there are lots of trees to cover the horizon line but in the case of Gravity Hill, you can still see the surrounding area fairly clearly. What can change someone's perception is also the trees that have grown in such a way to give off the illusion of growing "uphill" when actually being downhill, which makes this road even cooler as it does seem so realistic to the human eye as an uphill.

So how does physics play into this? Simply put, when the car is in a neutral position, it will continue to move forward due to gravity alone. For Gravity Hill, the slight downward slope is enough to make the car continue to roll forward even with factors such as the force of friction, drag, and normal force. Although this is fairly simple, it is interesting to see how impactful an optical illusion is in making one question the laws of physics!

Sources:

https://www.sfgate.com/local/article/I-went-to-the-most-confusing-road-in-the-Bay-Area-15631568.php

https://www.geographyrealm.com/what-are-gravity-hills/