# Did You Know?

M V Pattabhiraman

In 1665, the story goes,that Sir Isaac Newton,after seeing an apple fall, sought the reason and came to the conclusion that the natural force of attraction exerted by the earth upon objects at or near its surface tends to draw them towards its centre. The force of gravity is equal to Gm1m2/r2 where m1 and m2 denote the masses of the earth and the object and r is the distance between the centre of the earth and the object and G a constant called the constant of gravitation. The value of G has been found to be 6.673 x 10 -11 which is equal to 0.00000000006673. This implies that if the object is further away from the centre of the earth, the force of gravity on it is less than on objects which are closer to the centre of the earth.

Gravitation is a natural phenomenon by which objects attract each other. It is gravity which keeps the earth and other heavenly objects in their orbit. We take it for granted that it is easier to walk down a slope than up a hill and that one can drive a car down a slope by remaining in neutral gear. The idea that any object will fall down rather than go up is what we are usually accustomed to, but an experience I had on a recent visit to the United States, went against all these notions of gravity.

Bedford is a small town in the state of Pennsylvania in the US, about 50 miles east of Philadelphia. Driving in a wooded area, we stopped our car at the foot of a hill. When we put the car in neutral gear and took the foot off the accelerator we found the car rolling up the hill. How could we be going up looked around but there was no one. We returned to the town and asked people about this phenomenon and were told that this was a speciality of the place and many tourists came here to experience this phenomenon of gravity being defied.

We learnt that there were several places where one can encounter this kind of a phenomenon. In the USA alone there are about 30 to 40 such places. In Gansu province in China, there is a slope of 200 ft at an angle of 15 degrees where one can see a river running up the hill. There is also the famous Oregon vortex in USA where, in a circular spot about 165 ft in diameter, people appear to be shrinking as they walk towards you. The only spot in India where one can experience this phenomenon is in Ladakh.

Why does this happen? People living near the hill we visited tell a strange story. Years ago, when a school bus going up the hill suddenly stopped the children got down to push it up. But, the bus began to roll down and in the process some students were crushed and killed. The people believe that it is the ghosts of these children that push the vehicles up the hill! Nice story if one wants to believe in ghosts, magic and miracles. But is there a scientific explanation?

Yes. The answer – optical illusion. Experiments were performed to understand the phenomenon. The altitudes at the bottom and summit of one such hill were 1582 and 1569 ft., which implies that the hill actually has a downward slope. What is actually a downward slope appears to be uphill due to an optical illusion. A horizon that is either completely or mostly obstructed makes it difficult to judge the angle of the slope. Our sense of balance is completely overruled by the nature of the surroundings and what is actually a slight downhill looks like an uphill. Nature working its magic!
(Contributed by M V Pattabhiraman)