Please Don't Ban My Concrete Block Smashing Physics Demo From Schools

04/01/2015 03:17 pm ET | Updated Jun 01, 2015

Dear school administrators, parents, politicians, OSHA, and lawyers:

By now you've probably seen the viral video below of a concrete block smashing Physics demo gone wrong* (WARNING: disturbing content.) As a veteran Physics teacher who does this demonstration in classes several times a year, both as the "smasher" and as the "smashee," I am assuming that you are about to walk down to my office, email me, or draft a law telling me that I can never perform such a dangerous demonstration in my classroom ever again. Please don't. You shouldn't. Here's why.

1. From a Physics education standpoint, the demonstration is very valuable.

The cinder block smash is a classic demonstration of inertia and conservation of energy, two of the most important principles in introductory Physics. The large energy of the swinging sledge hammer is dissipated into the cinder block in via kinetic, thermal, and other forms of energy, rather than being transferred (painfully) to the person lying on the ground. Furthermore, the inertia of the massive cinder block prevents the block from experiencing any significant acceleration downward into the the smashee's chest. Done correctly, the most uncomfortable aspect of this demonstration is merely the weight of the cinder block resting on your trusted colleague prepares to swing. The actual impact feels no more significant that a friend's good-natured punch in the arm. When safely executed (see video of me performing the stunt at the bottom of this page), the demo is a powerful and memorable experience for the students. As a class, we are able to connect back to this demo throughout the year as we touch on energy and inertia. And when I cross paths with former students who have grown up and moved on, they often remark that they remember the demonstration vividly and that the associated physics concepts have stuck with them.

2. From a scientific literacy standpoint, the demonstration is very valuable.

Too much of our nation consists of climate change deniers, disciples of Dr. Oz, and proponents of intelligent design. Educators have an obligation to combat pseudoscience with scientific literacy. Understanding that the scientific method produces reliable scientific principles that can be trusted is a significant component of scientific literacy. What better way to demonstrate trust in science than to put one's own ribcage and internal organs on the line? It's important for my students to see that when I am lying on the classroom floor with a concrete block on my chest, I'm not worried about the physics principles holding up their end of the bargain--I'm only worried about whether or not my colleague with the sledgehammer is going to miss. Which leads me to reason #3:

3. When done properly, the demo is not nearly as dangerous as you think.

The teacher in the video made several big mistakes:

  • First and foremost, you don't straddle the smashee. You stand to his side and swing "perpendicular" to him so that if you miss you hit the floor. You never straddle for *precisely* the reason graphically depicted in the video.
  • Second, the teacher is using the wrong grip -- while a "baseball bat" style grip may be best for chopping logs, smashing blocks on top of humans requires control and precision (and less speed than you think.) The guy needs to choke up with his right hand about 30 centimeters.
  • Third, you've got to use a sledgehammer -- not an axe! The surface area of an axe -- even the back of an axe--is too small. It's more likely to "slice" through the block rather than smash, as it does to the edge of the block in this video. A true smash is required for the energy to be safely dissipated. Furthermore, you can let the large mass/gravitational potential energy of the sledge do most of the work, leaving your muscles and focus to be on accuracy rather than speed or power.
  • Fourth, the only person wearing eye protection in the video is the guy on the ground. Every single student along with the axe-swinger needs to be wearing goggles here. When we do this in my classes, sharp pea-sized fragments go *everywhere* at a high velocity. I mean, we have to sweep the far corners of the classroom afterwards. This is the most dangerous thing we do from an eye-safety perspective all year. It's shocking to me that there are no safety goggles present in this video. In a way, the teacher is lucky he *only* hit a guy in the crotch with an axe -- if he'd shattered the block properly he could have taken out some students' eyeballs instead.
  • Lastly, given the four aforementioned mistakes, I'm assuming these guys had never done this before. Their fifth mistake is not practicing once or twice privately, without students, so that if they screw up like this no one is around to film it and have it go viral.

Had the teacher avoided these mistakes, he would have had a demo that is actually much safer than it appears. My department has been smashing a half-dozen concrete blocks each year as long as I can remember and have never had an incident. That said, this *is* a dangerous demonstration and should not be taken lightly. Which leads me to:

4. A little bit of danger can be a good thing.

This demonstration gives teachers the opportunity to talk about risk management and risk mitigation. It also shows to students in a very real way how much we care about our students and how highly we value their education. And it provides us with an opportunity to demonstrate that Physics teachers are a special breed -- we may be nerds, but we are fearless nerds with nerves of steel.

*It should be noted that, according to various unsubstantiated internet comments, everyone involved in the video turned out okay, and no-one was actually fired.

Below: Video of me safely doing the cinder block smash demonstration on a colleague. (It's hard to see in the low-resolution video, but my students are wearing safety goggles and my glasses are ballistic-rated.)