Striking a Blow for Physics

Brian Welsch smashes a block with a sledgehammer on a student lying on a bed of nails.

When UW-Green Bay Assistant Prof. Brian Welsch wants to make a point in his Fundamentals of Physics class regarding Isaac Newton’s second law of physics, he makes quite an impact—with a sledgehammer, to be exact.

UW-Green Bay has not had a physics major in more than 20 years, but still offers a physics minor. This particular class generates interest from students in science and engineering, including premed and even future physical therapists.

It’s the type of class where according to Welsch, “you can do a lot of hands-on laboratory-type things.” Which, in this case, entails smashing a cinder block on the chest of Lab Manager Joe Schoenebeck, as he lays sandwiched between a bed of nails and a “chest of death” (also composed of a board of nails).

For those taking notes, the demonstration expresses Newton’s Second Law in terms of change of momentum, impulse and impact. It seems there are examples of the law all around—many with rather painful results. Take the typical falling mountain climber Welsch explains. “The falling climber picks up momentum, but if the change of momentum (as in hitting the rocks) is too short the net result is broken bones.”

The same principle is at work when demonstrating how an airbag absorbs the impact of a collision. But in this demonstration, the “airbag” is more “Flintstonian” in nature; using a cinder block as the airbag, and the sledgehammer representing the speeding object.

And the reason for a bed of nails on bottom, plus a “chest plate of death” (more nails) on top?

“Dramatic effect” explains Welsch. (Actually, the crumbling of block will also dissipate the impact of Schoenebeck’s body against the nails to the point that he won’t become a human pin cushion).

As Schoenebeck settles on to his bed, Welsch continues the lecture, “If I don’t do a good job crushing the airbag, not only will I crush Joe’s sternum, he’ll be impaled by 1,000 nails.”

“2,000” corrects Schoenebeck. (He should know, being the person who nailed them.)

“I just have to not miss,” Welsch assures the classroom. Not breaking the block means the full force of the sledge will be transferred to the “victim.” And if that happens, the concept momentum and impact will still be illustrated, but in a perhaps more painful way.

As for the results of the demonstration? Spoiler alert: the professor nailed it. Plus, as a bonus, a student worker also volunteered to get hammered.

Story by Michael Shaw, video by Sue Pischke

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