Ans : The answer lies in the difference between weight and pressure. Although the tractor is much heavier than the farmer, its weight is distributed over a much larger area of its bottom surface. Consequently, the load carried by each square centimetre of its bottom surface (the "pressure'0 is fairly low. On the other hand, the weight of the farmer is concentrated over a much smaller area of his feet, producing a much higher "pressure". An object penetrates deeper not because it is heavier but because it exerts a higher pressure (force per unit area) on its support.
2. Why do we find it easier to aim with one eye?
Ans : The crucial point is that while the combination of two eyes enables us to judge depth or distance, single-eyed vision gives us better sense of alignment. Suppose one is aiming at a target with an arrow. With both eyes open, one cannot keep the arrow aimed at a target because the straight lines from our eyes to the target through the tip of the arrow make a small angle with each other and intersect at the tip. So the two eyes give us slightly eifferent alignments. This is avoided by keeping one eye closed. However, the price we have to pay is the loss of the sense of distance or depth.It is therefore remarkable that Mansur Ali Khan Pataudi(who played for Sussex Country and later captained the Indian cricket team) was a good batsman as well as a brillian fielder in spite of having one-eyed vision.
To disgress a bit, aother curious thing is that a painting or photograph looks better with one eye. The effect is enhanced if we look through a tube. The reason is simple. When we look at a picture with both eyes from a moderate distance, we recognise it as a flat surface. But when we look at it with only one eye, our minds are free to take up the suggestions made by light and shade in the picture. So, after we have gazed at it with one eye for a while, it begins to acquire a three dimensional character.
3. Is there any difference between how birds and aeroplanes fly?
Ans : The key difference is that birds use their wings for both lift and propulsion, whereas aircraft use their wings for life and their engines for propulsion. How an aircrat generates its lift has been explained earlier. Birds' wings are the last words in aerofoil design, and aerodesigners have not yet been able to come up with wings that are both flexible and strong to achieve lift as well as propulsion. A bird also requires a strong heart to power long flights. The powerful heart is kept warm by feathers, one of the best natural insulators one can think of. Thus, equipped with a superb engine, the world's best wings and springy toes for a takeoff, a bird can soar into a graceful flight at a moment's notice.