Volume 1, Sunday, November 14, 1999

More about item 5 Time Toss in "How The World Works".

Concern for the teacher : For a ball tossed up, the position of the ball in the figure is not spaced uniformly. Why?

Answer: If a solid object (like a baseball) is thrown up, the vertical speed keeps on decreasing as the ball climbs to the maximum height. Every student will know that the speed will decrease on the way up. What some of them do not know is what "speed decreases" really means. It means that you cover even less distance for the same interval of time! The distance covered for the first interval of time will be more than the distance covered for the second interval. That is why you see bigger spacing between the positions of the ball for the first time interval, a little less for the next, and so on. On the way down, the spacing between the positions of the ball will increase because speed continuously increases.

Volume 2, Tuesday, October 3, 2000

More about item 38 Pull-Tabs in "How The World Works".

These are all examples of simple mchines.

1. Pull Tab of a pop-can
2. Nut-cracker
3. Wrench
4. Bottle opener
5. Inclined planes

Class Work in the order:
A. Give the lesson in the book as a reading exercise.
B. Let students bring: A full can, an empty (clean) can, and a pulled out pull tab.
C. Without using the pull tab, ask children to tear open the can. This will make them realize how hard it is to open the can without the tab. Now you may ask for ideas to open the can without the use of the pull tabs.
D. Try a few of their ideas.
E. Describe the pull tab, the pivot point, the short length, and the long end.
Let each student identify the pivot point or fulcrum.
F. Now ask them to draw a Pull-Tab with the fulcrum point. When they give you back the sheet, make sure you correct them (if they are wrong) by saying that two lengths are not equal. One is about 4 or 5 times longer than the other.
G. Again, identify the short arm and the long arm. Say .5 cm and 2 cm.
H. Find the multiplying factor (= 2/0.5). That is how much the small pulling force of your fingers is multiplied with.
I. Now tie a string on to the pull-tab of an unopened can. The other end of the string should be tied to a scale (20 lb scale).
J. Hold the can and pull on the pull-tab. When you hear the pop sound, note the reading on the scale. This is the force that we generally use to open the can.
K. Now multiply the scale reading by the multiplication factor. This is the force that we get at the short end.

Note: At the short end, it is just not the force alone that matters. Pressure also matters. The short end is a little sharp.