Photo by Pop & Zebra on Unsplash

Summer is here! That means swimming, barbecues, and watermelon. I’ve got to admit, though, I’ll be looking at watermelons a lot differently this season. 
Recently, I came across a Black & Veatch video illustrating the importance of wearing your hardhat. They did it by demonstrating structural bolt falling from 20 and 30 feet onto a watermelon. 
While physics is not my primary background, I thought it would be interesting to share Rhett Allain’s discussion of the video’s science.
Allain notes he’s skeptical of the video’s claim that the one-pound piece will have an impact force of about 2,000 pounds when it collides after falling 20 feet. He notes “it’s really difficult to calculate the impact force for a couple of reasons”: impact force is typically not constant plus impact force depends on the stopping distance. 
He suggests instead that the falling bolt problem is a “perfect situation in which to use the work-energy principle.” He goes on to discuss the many considerations such as the one pound bolt falling its distance, making contact with the watermelon and still moving some distance, and the backward-pushing force on the bolt. He puts it all together in a work-energy equation:
Then he considers impact force, and tries to determine why the bolt dropped from 30 feet instead of 20 feet smashes through the watermelon. He notes, “Honestly, I have no idea where they are getting their values for this video. (They probably need a good science consultant.)”
Clearly, in the video, the melon breaks. Its structural integrity is disrupted and it falls apart. It’s a gooey mess, and no one wants to think of the same thing happening to their head.
Allain points out also that a hard hat will increase impact force so that “if the bolt hits the hard hat and stops over a shorter distance, this would produce a higher average force.” Yet he also notes, “the hard hat does do one thing that’s very nice. Since the hat has a rigid surface, it distributes the impact force over a larger area, which reduces the impact pressure. Lower pressure means there is less chance that the bolt will penetrate your head.”
Ah, what a relief! Even if you don’t get the physics.

Key Takeaway

Ultimately, this video and Allain’s discussion had me thinking again about the importance of workplace safety. At the same time, Allain’s questioning the science demonstrated reminds me of my consistent warning against assumptions. We need to always be testing our thinking, whether it’s about filtration technology or busting watermelons. Be safe this summer!