Defying Gravity: DIY Physics Experiment

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Staring at a list of physics equations can be daunting. These mathematical expressions, though, are meant to model the world that we see around us. Luckily there are several do-it-yourself demos that can illustrate concepts such as circular motion. For example, swinging a bucket full of water over your head seems like a good way to get wet, right? Intuitively, gravity will make the water fall out—right onto the person underneath.

not-wet-gif-watermarkedI tried this physics experiment to demonstrate why our intuition is wrong. As shown by the gif, I stay dry. There is more to the story than just gravity because the bucket is moving in a circle at a high speed. The circular motion I make with my arm causes an inward centripetal force on the bucket. Meanwhile, the bucket wall constrains the water that would otherwise fly off tangentially from the arc to stay inside. Is centrifugal force – pushing outward during rotation – defying gravity and keeping the water in the bucket? No.

 

To be in the know if you haven’t taken physics, centrifugal force isn’t exactly “real.”[1] If you’ve ridden a roller coaster, then you have been analogous to the water inside the bucket. There seems to be an invisible force slamming you into the back of your seat while you’re whizzing around the tracks, but it’s really the seat pushing back on you to stay in. Centripetal force is occurring; you just can’t tell due to your frame of reference.

ck1.pngOf course, if you slow down enough at the top of a teardrop-shaped (clothoid) loop-de-loop you might feel a sensation of weightlessness as gravity takes over. A harness or bar will catch passengers from falling out of the rollercoaster.[2] In our experiment with the bucket, if you don’t swing fast enough then there aren’t any safety measures to catch the water! Thus we have a clue that an object maintaing circular motion and its velocity are related, which we can talk about using equations.

For those who have taken a physics course, this phenomenon might be review. However, thinking about how we understand everyday life is incredibly powerful. The basic principles of circular motion inform how we design rollercoasters, spacecraft, and who knows what next.FaSCInate_Logo_NoText.png

References

[1]. Lucas, J. (2015, October 15). What Are Centrifugal & Centripetal Forces? LiveScience. Retrieved from http://www.livescience.com/52488-centrifugal-centripetal-forces.html

[2]. Berry, N. (2014, March 24). Why Roller Coaster Loops Are Never Circular. Gizmodo. Retrieved from http://gizmodo.com/why-roller-coaster-loops-are-never-circular-1549063718

 

All images are courtesy of Sheridan Rea.

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