This Week's Experiment - #307 Bernoulli in the Shower?

a towel
a shower that has a shower curtain

My original idea was to get a towel wet and then spin it round and round. Inertia would cause the water in the towel to move outwards and fly off the towel. Not wanting to make a mess (which I would have to clean up), I stuck my arm and the wet towel into the shower. Closing the shower curtain as much as I could, I began to swing the towel round and round, faster and faster. The water was flying off the towel, just as I wanted it to, but the shower curtain kept getting in the way. It would move inwards and get hit by the towel. Why did the curtain keep moving inwards?

My first thought was the Bernoulli effect. Very simply that tells us that a fast moving fluid (A fluid is something that flows, not necessarily a liquid) will exert less sideways pressure than a slow moving fluid. The fast moving towel makes the air in the shower move faster. That would mean that it would not press as hard on the inside of the shower curtain. The air on the outside of the curtain would still be pressing just as hard as usual. Since the outside air was pushing inwards harder than the inside air was pushing outwards, the curtain would be pushed inwards.

Science books used to say that the Bernoulli effect was what made airplanes fly. The shape of an airplane wing was supposed to make the air travel faster over the top of the wing, causing less downwards air pressure on the wing. There was still just as much air pressure under the wing as usual, pushing the wing upwards just as the shower curtain was pushed inwards. When the wing is pushed upwards, so is the rest of the plane.

Sounds great, but in both cases the Bernoulli effect is not the whole answer. For the airplane, most of the lift is caused by air hitting the bottom of the wing. The wing is tilted, so that as the plane moves forward, most of the air hits the bottom of the wing. You can see this by holding your hand out the window as your car is moving down the highway. If you hold your hand flat and tilt the front edge slightly upwards, you will feel the air pushing it upwards. The collision between your hand and the air forces the air downwards and your hand upwards. At this point, your mother will yell at you and tell you to keep your hands inside the car.

With the spinning towel, there is also more to the story. Step inside the shower and close the curtain. Face towards the curtain and spin the towel. Pay close attention to where the curtain moves inwards. The strongest pull seems to be in the center of your spinning towel. If the inward pull was from the Bernoulli effect, then the pull would be strongest at the far end of the towel, where the air is moving the fastest. Why is the strongest pull in the center?

Spinning the towel causes the air inside the shower to spin as well. The spinning air moves outwards, just as the water does. As the air moves outwards, there is less air pressure left in the center. Surrounding air is pushed in towards this low pressure area, and so is the shower curtain.

Since it is science fair time in many areas, I thought this was a very good experiment, since my original hypothesis was wrong. Does that mean my experiment was a failure? No. I learned more from guessing wrong. The same is true for your science fair project. It is perfectly fine for your hypothesis to turn out to be wrong, as long as you find out why and learn something in the process.

These experiments are from Robert Krampf - The Happy Scientist

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