Question Video: The Relationship between Pressure and Surface Area

Why should you roll or belly crawl rather than walk across a thinly frozen pond?

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Video Transcript

Why should you roll or belly crawl rather than walk across a thinly frozen pond?

Now let’s imagine there are two people racing to get across a pond. On the other side is a box of treasure. And for some reason, they aren’t able to go round. The pond is frozen over with thin ice. When the first one steps onto the ice, it cracks and he goes splashing straight through the ice. While the first person is splashing around and trying to swim for shore, the other one scrambles on his belly all the way across the pond to the treasure. So, the question is why is rolling or belly crawling better than walking on thin ice?

If you walk, the smallest area that’s going to be touching the ice is the sole of one shoe. Meanwhile, rolling across the ice means that there’s much greater area in contact with the ice. The pressure exerted on the ice is equal to the force divided by the area. If two people weigh the same, they’ll exert the same force when standing still. However, the area they cover will be much smaller if they walk rather than roll.

So, the pressure on the ice is much greater while walking than rolling or doing a belly crawl because the area in contact with the ice is much much smaller. The higher the pressure on the ice, the more likely it is to break. So, you should roll or belly crawl rather than walk across a thinly frozen pond because the greater the contact area with the ice, the lower the pressure and the risk of it breaking. Of course, if you have the option don’t go out on thinly frozen ice to begin with.

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