A small amount of puffed rice is mixed with chana dal. Can you think of separating the mixture by any method other than handpicking?
Answer
Yes — winnowing (blowing air) is the best method here.
Compare the two components:
Component
What it is like
Puffed rice (murmura)
Full of air, very light for its size, floats on the slightest breeze
Chana dal
Hard, solid and comparatively heavy
Put the mixture in a soop or a flat plate.
Toss it gently in front of a fan or in the breeze.
The murmura is blown aside; the chana dal falls back into the tray.
Why handpicking is a poor choice here: handpicking is convenient only when the unwanted component is present in a small quantity and the pieces are big enough to pick up. Grains of murmura are many, light and crumble when pinched — you would be at it all evening.
Another workable method: sieving, using a sieve whose holes let the chana dal through but hold back the bigger puffed rice. Winnowing is still quicker.
Q2.
How do you think farmers separate so many grains from husk?
Answer
They let the wind do the work — the method is called winnowing.
The threshed heap (grain and husk together) is scooped into a soop, a shallow tray woven from bamboo.
The farmer stands on a raised platform so that the falling material has a long way to travel through the moving air.
He tosses the material upward, into the direction of the wind, again and again.
Grains, being heavy, fall almost straight down into a neat heap. The husk, being light, is carried sideways and settles far away.
Why the raised platform matters: the longer the material stays in the air, the longer the wind has to push the husk sideways. From a low height the two would land almost together and the separation would be poor.
Did you know? A modern thresher machine does threshing and winnowing at the same time — it beats the stalks and then blows a strong jet of air through the falling grain.
Q3.
What do you infer from Fig. 9.4? Do both the components, wheat grains and husk, fall at the same place? Which of the two components get blown away? Can the wind separate the two components?
Answer
No, they do not fall at the same place. The husk is blown away and lands far off; the grains fall close to the farmer’s feet. Yes, the wind can separate the two components — that is exactly what the figure shows.
Winnowing: the same wind acts on both components, but it can move only the light husk. The heavy grains drop almost straight down and form a separate heap.
Why they land in different places: the wind gives the same sideways push to everything falling out of the tray. A light piece of husk is easily shifted by that push and drifts far. A heavy grain hardly notices it and falls nearly straight down. Two heaps therefore form — grain near the platform, husk further away.
Tip: This is why winnowing fails indoors. In a closed room with no moving air there is no sideways push at all, and both components land in the same place.