Subject:  [B*E*S*T]  Testing the models
Date: 1997-11-19 20:36

Susan Seyer wrote:

>The Morris Park 4th graders are putting final touches on their tabletop
>cars (all 9  teams!) with a lot of help from our friend Ben Kelly...thank
>you for all of your efforts, Mr. Kelly!!

Great job! Now comes the fun part; testing them. Have you a plan so they
can test their cars against each other, and pick the best design?

A simple test is to roll them down a ramp, and see which one goes the
farthest. What effect does adding weight have? How many wheels are best?
How hard is it to get them to roll straight? This way, they have some
objective data, rather than opinions (this one looks fast, but is actually
slow when we measure it).

The designs become self-grading. The class wants the best design, no matter
whose it is. They will have to make the transition from "my design vs. your
design" to "our design".

They can also measure how fast they are going; speed = distance / time.
Measure the distance in feet for example, and the time it takes to travel
that distance in seconds. You'll probably need a stopwatch, or watch with
start/stop capability. They will have to make several measurements, because
of "experimental error".

Feet / seconds = feet per second; a measure of speed. For example, it might
go 5 feet in 1 second. They can convert feet per second into miles per hour
like this:

 5 feet     1 mile     60 seconds   60 minutes   5 x 1 x 60 x 60    18000
 -------- X --------- X ---------- X ---------- = ---------------- = -----
1 second   5280 feet    1 minute     1 hour      1 x 5280 x 1 x 1    5280

We have FEET per second, but want MILES per hour. How many feet in a mile?
1 mile = 5280 feet. So we multiply by 1 mile and divide by 5280 feet.
Notice that we have feet in the top (5 feet) and in the bottom (5280 feet).
The dimension "feet" cancels because it's in the top and bottom.

We have feet per SECOND, but want miles per HOUR. This takes two steps. 60
seconds = 1 minute, and 60 minutes = 1 hour. So we multiply by 60 seconds
and divide by 1 minute, then multiply by 60 minutes and divide by 1 hour.
As before, the seconds cancel (one in the top and one in the bottom), and
the minutes cancel (same thing).

What is left?  Multiply everything on top. Multiply everything on the
bottom. Divide the top by the bottom. You have miles per hour, because
"miles" and "hour" are the only names left in the top and bottom.

 5 x 1 x 60 x 60     18000 miles
 ----------------  =  -----------  =  3.409 miles per hour
1 x 5280 x 1 x 1      5280 hour

I know; I don't expect 4th graders to "get" it yet. But you can expose them
to the concept. Then give them a worksheet with places to write in the
numbers, and they can do the math with a calculator. So they can run a
test, get the feet and seconds, and calculate how fast it went!

I hope this helps.

Lee Hart