# Fun with postcards

ACTIVITY TIME
Last Updated 17 November 2011, 17:42 IST

A postcard is a rectangular piece of thick paper or thin cardboard intended for writing and mailing without an envelope. Postcards were introduced in India for the first time in 1879.

The postcard still remains the cheapest mode to send a letter from one place to another anywhere in India. The postcard is highly subsidised. You buy it for 50-paise but it takes the government an average of Rs 2 to print process and transport it to your desired destination. A postcard can be bought in any of the 1,50,000 post offices spread all over the country.

As postcards are mass produced their length, width and weight is a standard. An ordinary postcard is 14-cm long and 9-cm wide. An old postcard comes in very handy to estimate 6-inches (15-cm).

The average weight of a postcard is very close to 2.5gm or 2500milligrams. As the area of the postcard is 14 x 9 = 126 cm2 so 5cm2 of the postcard is a good estimate for 0.1gms. And a 1cm2 of the postcard is a pretty good estimate of 20-milligrams. So, with the help of a humble postcard you can make micro-weights too.

Tape the 9cm edges of three old postcards to make a prism, a rectangular pipe and a circular pipe. Each structure will be 9cm high. Which structure is the strongest? Which can take the maximum load? Test it by balancing books on top. The circular pipe will be able to sustain the maximum load of over 4kgs of books before getting crushed by their weight!  Why is the circular pipe the strongest? Here the triangle, rectangle and circle have the same perimeter of 14cm but the circle has the largest cross-sectional area and therefore the load per unit area is the least. Nature is the greatest designer and now you will appreciate why tree trunks are circular and not triangular or square!

WHICH HOLDS MORE?

Fold two cylinders from two postcards by bringing their edges together and taping them. Fold the tall, thin cylinder by taping the 14cm edges together. Fold the fat, short cylinder by taping the 9cm edges together. Both cylinders will have the same surface area because they are made from similar postcards. Ask your friends: “Which cylinder will hold more sand?” Most will say that both cylinders will hold the same amount of sand. But on testing they will be in for a surprise.

Fill the thin, tall cylinder with sand to the top. Then slide the fat cylinder on the thin one. Finally shake the thin cylinder and remove it. This way you can easily compare the volumes of sand they contained. The fat one will be only 2/3rd full! Why? The volume of a cylinder depends on the square of the radius and its height. As the fat cylinder has a larger radius, the square of the radius makes a lot of more capacious. Watch video http://www.you­tube.com/wat­ch?v=wCXp0­zvGUKc

POSTCARD STRUCTURES

Place a flat postcard across two blocks such that only 1cm of the ends rest on the blocks. Place a fat 5-rupee coin (weight 5gms) in the centre. The weight of the coin will bend the card down. On placing another 5gm the card will buckle and fall down. Now make pleats and fan-fold the postcard. The corrugated postcard can easily support a weight of 100gms. Fan-folding the postcard transforms it from a material to a structure – and the pleats make it strong. Many houses have corrugated tin roofs because they are much stronger than flat roofs.

WALKING THROUGH A POSTCARD

Can you walk through a postcard? Seems impossible, but actually it’s quite easy. Fold a postcard in half along the length. Then make alternate cuts along the top and bottom edges as shown. Finally cut the top folded from point A to B. Then gently open the postcard and to your will be pleasantly surprised that it has become a circular wreathe – long enough for you to walk through it!