Strongman tricks often involve seemingly impossible tasks like bending iron bars, lifting gigantic weights, or pulling trains with your teeth. This “teacher trick” doesn’t take great strength to impress. Using a clever arrangement of a mirror and a laser, students can see that steel girders, concrete block walls, and other seemingly immovable materials flex when a force is exerted on them, even a force as small as the force exerted by a wimpy pinkie finger.
Most people don’t think at all about how “rigid” objects exert forces on other objects (the force that teachers often term “normal” force”). Early in a unit on Newton’s Laws, it would have been asserted that the downward force of the earth on an object is balanced by an upward force of a surface on the object when we observe that the object is at rest. The terminology that is often used for this situation is problematic. Following Newton, the force of a surface on an object is often called a “reaction force.” But how can a table “react?” What do we mean by reaction? This is an issue that is often raised by students. If they don’t bring it up, I pose the question myself. This trick supplies the answer and the explanation.
The script goes something like this:
“Students, suppose I place a book on a table, and then ask you: “With how much force does the table support the book?” You would have no problem stating that since the book is at rest, the table must produce an upward support force equivalent to the downward pull of the earth on the book (the “weight” of the book). But what happens when we replace the book with something else? Now suppose I put a watermelon on the table. The same logic tells us that the table supports the melon with a force equivalent to the weight of the melon. A garbanzo bean? A feather? A fifty-pound catfish. . . Hmm. That’s a clever table. It always knows what to do. How did the table get so smart?”
At this point, I would likely pause, ask them to talk to their neighbors about “smart tables.” We discuss a few of the ideas (typically, nobody has a complete answer, but everybody agrees that the question is ridiculous). –Of course tables aren’t smart. They just “react” to what you do.
“But how does a table “react?” Wouldn’t that be interesting to know? Maybe simple things are more mysterious than we realized?
I make a proposal. “Let’s make a model table and see if we can observe how it “reacts.” The model table is constructed of light wood strips or meter sticks and some books.
I place something light on the model table (a feather or a marshmallow, say). “Did anybody see it react?” -No. And yet, it must have “reacted,” whatever exactly we mean by that. Then I place something heavier on the table, say a large mass, or a big physics book. We might see a slight sag in the table at this point.
Next, something even heavier, until there is an unmistakable sag in the “table:”
I could continue, adding mass until the table breaks, but it’s easier on meter sticks to just suppose we put something really large on the table. Suppose an elephant came in and sat on the table. What would happen then?” -A normal table would break. Tables have limits.
“How does the model table know how much force to exert on the objects? ” -It sags. “Is it possible that the table or the floor sags when you put something on it?” –Maybe. “Could you ever see it sag?“ –I doubt it. “What if I could somehow magnify the movement? Would you believe it then?” –Maybe. “Would you believe I can make a steel girder (or a concrete block wall) move by just pushing with my little finger?” –Nope.
At this point I set up the materials to show the movement of a girder, or concrete block wall:
The laser light is reflected off of the fragment-of-CD mirror so that a visible spot is seen on a wall or ceiling several meters away. A tiny movement of the girder moves the metal rod (“attached” to the girder or wall with a lump of modeling clay). Friction between the metal rod and the dowel rotates the dowel a tiny amount. This changes the angle of the CD, and the reflected spot of laser light moves. If you push on the girder and then pull on it, you can see that the movement of the laser light changes direction. Here is a picture of one of my students getting ready to push the girder:
And here is a video of the laser spot moving. The motion is not so pronounced, but is usually still evident if you push and pull with a pinkie finger.
It’s even more impressive if you start out with the apparatus in a room connected to a concrete block wall, and then send someone outside to push from the other side.
At this point most students can explain that objects, no matter how rigid they seem, “exert” forces by deforming slightly. The greater the force, the more they deform.
When we go back into the room, I pull out the Pasco Matter Model:
The red plastic spheres represent simplified atoms/molecules, and the metal springs represent simplified bonds. This gives students a visual for what is going on when rigid materials exert forces. It is greatly simplified and quite a bit exaggerated, but also very memorable.
I could have pulled out the model from the start and said “Today I’m going to show you how matter behaves.” But, that wouldn’t be very dramatic, would it?
Physics Teacher Notes Below
This activity aligns with the College Board’s AP Physics 1 Learning Objectives 3.C.4.1 and 3.C.4.2:
|3.C.4.1: The student is able to make claims about various contact forces between objects based on the microscopic cause of those forces. [SP 6.1]|
|3.C.4.2: The student is able to explain contact forces (tension, friction, normal, buoyant, spring) as arising from interatomic electric forces and that they therefore have certain directions. [SP 6.2]|
I usually schedule this activity two or three meetings into the force unit. In class we would not have done much more than mention Newton’s Laws. The students would have learned how to name forces (using agent-object notation), and how to draw force diagrams.
I first learned of this demonstration from a fellow participant named Steve Brehmer (a physics teacher from Minnesota) at the Project PHYSLAB teacher workshop in Portland, Oregon. The format of the activity itself was inspired by the book Preconception in Mechanics by Charles Camp and John Clement, published by American Association of Physics Teachers. The 2nd edition is available in print from the AAPT Store or as a download from American Modeling Teachers Association when you join and gain access to their curriculum repository. If you buy the book, you’ll see that they have a more detailed strategy than I outline here.
If you would like to know exactly how much your wall (or girder) moves, I recommend this article from The Physics Teacher, “Demonstrating and Measuring the Flexure of a Masonry Wall: by Daniel MacIsaac and Michael Nordstrand.