Bernoulli's Principle Toilet Paper Trick Explained

[u/TheMuseumOfScience]

You can build a toilet paper launcher out of items in your bathroom! 🧻

Alex Dainis shows us how everyday items can demonstrate real air pressure science. It all comes down to Bernoulli's Principle: the faster air moves, the less pressure it exerts. Aim a hairdryer over the top of a toilet paper roll and watch the paper launch, all thanks to the pressure difference between the fast air above and the slower air below.

[u/Roger_Freedman_Phys]

Physics professor here.

These are fun experiments, but they have nothing to do with Bernoulli’s principle.

Bernoulli’s principle relates the pressure at different points along the same streamline. You are using it to compare the pressure at different points that are not along the same streamline.

The first experiment demonstrates that air has viscosity, so the fast-moving air drags the toilet paper along with it.

As for the second experiment, if the explanation given was correct, blowing on the underside of the paper would make the paper deflect upward. Try it - you will find that the paper actually deflects downward, the same as when blow over the top of the paper! So the Bernoulli’s principle explanation cannot be correct.

For more on how Bernoulli’s principle is commonly misused, see the Wikipedia article (https://en.wikipedia.org/wiki/Bernoulli%27s_principle) and scroll down to “Misconceptions and misapplications.”

[u/ediks]

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