In the classroom, I can take a quick step forward, and the ball will swing towards my chest. Same thing, but now with a pingpong ball taped on. Again, if I take a quick step forward, the pingpong ball will swing towards my chest. If I want to prevent this by gripping the vertical part of the coathanger, it will be easier than trying to do this with the golfball. Now I connect the two. What will happen when I step forward now? Since the golfball is much heavier than th epinpong ball, the golfball 'wins': the golfball will go towards my chest, and the pingpong ball will be forced forwards.
(More to come...)

• Galileo, Newton and inertia
• How do flies fly?
• How do flies land on the ceiling?
• http://www.physlink.com/Education/AskExperts/ae554.cfm">xxx
• balloon in a car
• balloons in a car
• balloons are full of holes
• helium balloons
• helium balloons

I started with an example a little closer to home: in the ceiling of the school's lobby there are these giant exposed wood beams. (I actually handled every one of them myself). How would you figure out the weight of these beams? I introduced the idea of density, the weight per unit volume of a material. I still had a block of wood that had been cut off one of the beams, and we measured it (6cm × 25cm × 25cm), and weighed it on my kitchen scale (1650 g) . The density then is:

               weight     1650 g
density = -------- = ---------   = 0.44 g/cm3
volume     3750 cm3

Each one of these cm³ weighs 0.44 grams, so the whole beam weighs 630000 × 0.44 = 277000 grams, or 277 kilos, or 600 lbs. (In 1997, the wood was much greener, and the beams weighed more than 800 lbs each).

You can see we did lots of unit conversions, which takes some patience in 6th grade. To give them a feeling of what grams an kilograms are in real life, I had brought my standard 1-gram weight (a dollar bill), and 1-liter bottle of water (=1 kilogram).

Now we did the same for the earth. The core of the earth consists of iron, and the outer crust is of course mostly rock. In between is the mantle, composed of molten rock. Using the same procedure as with the wood block, we measured the density of a slab of iron I had brought in, and of rock (a chunk of local sandstone). The density of iron came in at 5.7 g/cm³, and the density of the stone was about 2.7 g/cm³. So the average density of the Earth is somewhere between 2.7 and 5.7. Since the crust only makes up a small portion, and there is compression of material as you go deeper and deeper down, it turns out that for the whole Earth the average density is about 5.5 g/cm³: our tabletop measurements were in the right ballpark.

Next, we needed to find out the volume of the Earth. Luckily, the kids in this class knew the story of Eratosthenes of Alexandria. I was impressed! Eratosthenes had figured out the circumference of the Earth (see the links below), and got the answer correct at about 40000 km. That means the diameter is 12000 km. What is the volume of a sphere if you know the diameter? I used the approximation that it is about half of the volume of a cube that just fits around the sphere.

We were starting to deal with too many zeros, so I showed how to write the number of zeros in scientific notation. So the diameter of the Earth was now written as 12×10³ km, or
12×10⁶ m, or
12×10⁸ cm.
The volume of a cube that size is 12×12×12×10²⁴ cm³, and the volume of the Earth is half that, or 850×10²⁴ cm³.

Weight is volume times density, so

Weight of the Earth = 850×5.5×10²⁴ grams, or
4675×10²⁴ g, or
4.7×10²⁴ kg, or
10×10²⁴ lbs

As a final touch, does a number with 24 zeros have a name? Starting with our knowledge of millions and billions, we made the following table:

Therefore the approximate weight of the Earth is ten septillion pounds.

• Sizes of the Earth, Jupiter and the Sun
• Erastosthenes of Alexandria
• The world is round!