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DMS Graduation 1998:
An Experiment with Inertia

This is my talk for the Dunn Middle School graduation ceremony on 5 June 1998, delivered outdoors on the steps of the Middle School. I explain how to do it at the end.

I should explain that it's my custom to do a science experiment for my graduation speech. It's always a challenge to come up with an experiment that will work outside for a large audience, and will generate a message for the occasion. This year inertia almost got the better of me. I thought to repeat a graduation experiment from eight years ago. I figured that was long enough! Just last week, I realized that Laurel's sister Ari was part of that earlier class. And it just wouldn't do to repeat an experiment for the Coats family. Inertia was on my mind, so I will do an experiment today involving inertia in a physics sense. I'm going to push a nail through a board with my hand.

But first I have a little experiment to show what inertia is. It's always important to have the right laboratory equipment.

{Hold up toilet paper roll.}
Actually, this is the sort of lab gear I prefer. It doesn't intimidate, and it shows us that we already know a lot of science first hand, at least if we can ask the right questions to bring it out.

Toilet paper doesn't come with an operating manual (thank goodness!), but there is some skill involved in just getting it from the roll. If I pull slowly, it just keeps coming from the roll. To tear it, I have to give a quick jerk.

{Put TP roll on a short pipe to hold it, and demonstrate pulling.}
That's inertia. It's defined by the first part of Isaac Newton's First Law, "An object at rest tends to stay at rest." Newton goes on to add that "An object in motion tends to stay in motion." We'll get to that later. A slow pull gives the TP roll time to get moving, and the paper comes off. But a quick pull tears because the big roll tends to stay at rest and so resists the sudden force. This resistance is what we mean by inertia. It's a fundamental property of mass.

This explains one of those things we learn young, that it's hard to tear the paper at the end of the roll. There's not much mass. so the paper runs out all over the floor instead of tearing.

{Demonstrate pulling with a small roll of TP.}
There's also a stumper here. Does toilet paper work the same way in zero-gravity? I think so, since inertia and weight are different things.

Newton's Second Law takes the idea of inertia a step further. You might remember this as the equation F = MA, Force equals Mass times Acceleration. It makes more sense to rearrange this equation. Acceleration is really change. So A = F/M, or Change equals Force divided by Mass. Force and Mass are opposed here as the top and bottom of a fraction. A bigger force makes a bigger change (that's the top of the fraction), but a bigger mass makes a smaller change (at the bottom of the fraction). Four is bigger than two, but 1/4 is less than 1/2.

Here's a more elaborate demonstration. A weight hanging from the beam with a string above and a string below. What will happen when I pull on the string? If you think about toilet paper, you can probably guess the answer. (This might be bad advice in the middle of a long graduation ceremony!)

{Hang up weight with strings, place pad underneath.}
If your answer is "It depends on how you pull it," then you've got it.

First I'll pull slow and long. {Do it.} The string breaks on top of the weight because the slow pull gives the weight time to move and pass the force to the string above.

Now I'll pull short and hard. {Do it.} The string breaks below the weight this time. There's lots of force (that's the top of the fraction), but the big mass on the bottom of the fraction results in a small acceleration or change. The inertia of the mass absorbs the force to the point that the lower string breaks first.

(Note how it usually breaks on the knots. This is a real concern for rock climbers when they are the mass on the string!)

We usually use the idea of inertia in a negative way related to the word "inert". It means sluggish, hard to get started. But the concept also has a positive sense that's more interesting. Listen to Newton's First Law again. "An object at rest tends to stay at rest, and an object in motion tends to stay in motion." Moving things keep moving, unless you work to stop them.

You know this first hand if you've ever walked away from a parked car and then turned to see it starting to roll because you forgot to set the brake. You run to the car and push against it, but it's really hard to stop because it's so massive, even though it's slow. That's inertia too.

In a way, inertia is it's own opposite. It's hard to get something moving, but then it's hard to stop.

I'm going to demonstrate this by pushing a nail through a pine board. There's no trick here. I'll hold the nail in my hand and give it a good swing to generate lots of motion. The inertia of that motion should carry the nail through the board.

I should probably caution here -- Kids, don't try this at home. At least be very careful! Hands are too important and too complicated to risk. If you're not careful, you're likely to learn about Newton's Third Law, that the table and the nail will push back against your hand with an equal and opposite reaction. Don't use a finishing nail, and be sure to protect your hand with a thick pad. I'm using a small dish towel.

{Psych yourself up and do it! It helps to wax the nail.}
The secret is not to hesitate. This is like a game of nerves we sometimes play on Hike Club. Stinging Nettles are a local plant that has tiny hairs with an intense stinging toxin that you get if you brush against it and the hairs break off in your skin. But if you reach out and grab the plant hard, you crunch the hairs and don't get stung. If you hesitate just before grabbing, you're done for. (Fortunately an instant remedy, Curley Dock, nearly always grows nearby. Just crush a leaf and hold it against the sting.) Don't pull your punch. I'm sure this is how Karate masters can break boards. There's no trick to this. It's just mass and inertia and physics.

That's my experiment. So what's the point? We're here for a graduation, not a lesson.

We think of inertia as something we have to fight, but it's also something we can use. Once an object -- or a person -- gets moving, it's hard to stop.

Dunn Middle School is a place where we generate lots of good positive inertia. With school projects and assignments, and sports and Hike Club and bike rides, and music and dance and art, and most important, with the extra boost we get from friendships and mutual aid and high standards, we get used to doing things here. We call it Carpe Diem, seize the day, go for it and keep going.

As Mr. Newton says, an object in motion tends to stay in motion. You graduating 8th graders -- keep on rolling! Use your inertia. It will take you to new places, and you will do impossible things. Carpe Diem!


How to do it:

What I used for the speech:

The toilet paper trick is easy, and guaranteed to get laughs. Don't pull too sharply on the small roll. You want it to unwind. A grandparent came up after graduation and said they'd think of me everytime they used the bathroom. Other people think of me whenever they pass a roadkill. That's fame!

I used kite string and a 2 1/2 pound weight from a barbell set. Tie two lengths of string on opposite sides of the weight, and put a loop on one end so you can hang it. I prepared two weights so I wouldn't have to retie it during the speech. Our graduation is outside under a porch, so I hung it from from an eyehook in a roof beam. Put some sort of mat underneath the hook to break the fall. Do what you can. Be sure to practice this before hand. You need to find a balance between the weight and the strength of the string. This is a familiar demonstration, and special weights with a hook on each end are actually sold for it. It's described in many places, e.g. as demo C.7 in Robert Ehrlich's Turning the World Inside Out, Princeton University Press, 1990.

It really is possible to drive a nail through a 1-inch pine board with just your hands! It definately helps to wax the nail with some parafin. It also helps to use a coated sinker nail. Protect your hand with a thick pad made from a folded dish towel or bandana. Hold the cushioned nailhead firmly against the bone at the base of your middle finger. Then grasp the wrist of that hand with your other hand, raise both arms above your head, and slam the nail down firmly with a full arm swing. Don't hesitate! You will need a strong table to swing against, and maybe another board to protect the table. When I did it, the nail went through the board and 1/2 inch into the table below. It was pretty impressive to have to pry the board off! It takes some practice to hold the nail at the right angle so it enters the board vertically after the swing. I found this stunt in an old book, Science Magic by Kenneth M. Swezey, published in 1952 by McGraw-Hill.


last modified .

Marc Kummel / mkummel@rain.org