On the Direction of Circular Motion

Dileep V. Sathe
2 Prerana Apts, A-40, Kasturba Society, Dighi Post, Pune, MH, 411015, INDIA
Email: DileepSathe@... Phone: [020] 2669-6965

Following question has been included in some question papers of coaching
classes, training HSC students specially various CETs. For example, see MHT-CET
2006 paper of the Vijeta Academy, Kolhapur, Sakaal 15th April 2006, page 13.

A body is revolving with a constant speed along a circle. If its direction of
motion is reversed but the speed remains the same, then, A] The centripetal
force will not suffer change in the magnitude, B] The centripetal force will
have its direction reversed, C] The centripetal force will not suffer any change
in direction and D] The centripetal force would be doubtful. The correct answer,
as given by the Head of Academy, is A.

Coincidently, this question is concerned with a global and chronic problem in
physics education - that is the global tendency among students of choosing the
forward force as resultant force, acting on a body in uniform circular motion,
and ignoring the centripetal force. My 25-year long research is concerned with
the same and therefore I would like to draw attention of all science teachers,
not only physics teachers, to some key issues.

I have shown that the present treatment of circular motion is not logically
perfect and conflicts with other concepts like that of work and the law of
parallelogram. Albert Einstein's GR originates in the logical incompleteness of
Newton's laws of motion. But, my work shows that that logical incompleteness has
not been completely understood - as yet - and therefore we are still facing
grave problems in teaching / learning Newton's laws of motion, although they are
known for 300 years. Due to this conflict a large number of students tend to
give the wrong answer. Since this answer is wrong, as per teachers, the conflict
between teachers and students latently drives students away from physics. In
fact, this tendency has grown to such an extent in recent years and there it was
essential to declare 2005 as the Albert Einstein year for the sake of
popularization of physics.

The problem of direction has hardly received due attention of educationists and
physicists in the past because the reversal of direction of motion is never
considered in the teaching of uniform circular motion and therefore the above
problem will be provocative. This why I have been discussing my work with
physicists, rather than educators. For more information, see the text of my
lecture, which was dedicated to the memory of Prof Abdus Salam in a symposium in
samarkand, UZBEKISTAN, August 1999 -
http://www.iupac.org/publications/cei/vol2/0201x0026.html

Finally, I would like to narrate my own experience of 16th January 1993 in a
symposium in N Wadia College, Pune, I asked the audience to make a trajectory of
the body on which a centripetal force was acting. It was taken from the PSSC
Physics, 7th Edition, 1991, Kendall-Hunt Publishing Co, USA. And, as per my
expectation, some young people [all MSc, Physics] drew anticlockwise trajectory
and some did the opposite. This was quite useful for me why the comprehension of
circular motion is still in global trouble - though it is known for more than
300 years.

So, I request all science teachers, past and present HSC students to consider
this problem earnestly and send their views to me - on options given in the
questions and on any relevant aspect of circular motion and other related topics
like the planetary motion and Bohr's theory of hydrogen atom.

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