National Science Content Standard A: As a result of the activity students will develop the ability to do science inquiry.
National ESL Goal 2 Standard 2: Students will use English to obtain, process, construct, and provide subject matter
information in spoken and written form.
National ELA Standard 1: Students will read, write, listen and speak for information and understanding.
Objective: Students should be able to demonstrate the principle of the center of gravity.
Materials: (one each per student or group)
1-2 x 1 x 4 Styrofoam block (a wood block or a ball of clay will work as well)
7- nails (one nail goes in the block, the other 6 are given to the kids)
1-ball of clay (size doesn’t matter here)
1-pen or pencil
1-glass (plastic will do, though you may need to fill it with water for added mass)
1-fork and spoon (again plastic will be fine)
For you a match or a lighter will add even more WOW
Challenge the students to balance all six nails on the one nail in the block. Give them only a few minutes because as
soon as one student figures it out (and some might) many others will follow. Hopefully, this will not happen, instead you’d
like them to have a burning desire to learn how to figure it out, so DON’T tell them how to do it – go straight into the
exploring of the center of gravity. Then give the students another opportunity to solve the puzzle at the end of the
exploring and explanation sections.
Have students take their clay and roll it into a ball. Have them place it on one end of their pencil. Ask them to hold the
pencil with their thumb and finger with the clay ABOVE their “support point” (keyword for later discussion). Then have
them (or their partner) push slightly on the clay ball.
“What happens?” (It should be easily pushed over-so that now the ball is below the support point)
Now have them hold the pencil so that the clay ball is BELOW their “support point”. Have them push on the clay ball
“What happens?” (no matter how hard they push the clay will return to its original position).
Every object has a center of gravity. The center of gravity is the exact spot on an object where there is the same amount
of weight on one side of the spot as there is on the opposite side. In the case of the clay the center of gravity is near, if
not in, the clay (the heaviest part of the whole clay pencil thing).
In general, if you support an object below its center of gravity it is not very stable. If you give it a push it will fall over.
This is what happened during the first try with the clay and pencil. If you support an object above its center of gravity, it
is very stable. Always going right back to center.
Now for the bar/restaurant trick:
Have students take their glass, toothpick, spoon, and fork, place them on their desks in front of them. They need to
follow exactly what you do:
(if you want to do the match thing, your glass must be glass)
Place the glass in front of you
Gently work (or shove) the tines of the fork and the spoon (the part you put in your mouth) together.
Put the toothpick in between the two bottom tines of the fork.
Carefully balance the fork on the edge of the glass (you may have to work this for a few seconds to find the balance
point, its usually about half way on the toothpick).
Check to see how the students are doing.
Ask them where the support point is (where the toothpick meets the fork), since this is stable, where is the center of
gravity (below the support).
“Something that hangs down can be very stable as long as the support point is above the center of gravity.”
Now if you want the real WOW: Take your lighter or match and light the end of the toothpick. It should burn all the way
to the edge of the glass. It will appear as if the spoon and fork are being magically supported. Ask why it is still stable,
by now they should know it’s NOT magic.
The evaluation for this lesson is to see if students can put into practice what you’ve just discussed. This time many of
the students should be able to figure it out how to balance the nails without any assistance. Picture to help them if they
just can’t get it.
Have students locate their center of gravity. Line ‘em up against a wall (heels all the way against it). Lean forward.
When they fall over or have to go on tippy toes, the center of gravity has shifted. Hint: Girls can do this better than boys
because their center of gravity is lower.
The first nail goes in the styrofoam, as pictured. The rest of this is best built first then moved to the head of the
1. Lay one nail down on the table
2. Criss cross four nails so that the heads are resting over the shaft of the nail on the table
3. Place your last nail on top of the others with the head at the other end
4. The four criss crossed nails should be captured between the shaft of the other two nails
Adapted by Robert Harington