Trying to Teach Dynamics in a Reasonable Way : A Suggested Method for Schools and Universitties

Abstract

Isaac Newton understood why planets circled around the sun instead of travelling away in straight lines. They were all falling towards the sun just as smaller objects near earth fall towards the earth. Newton could not explain how the sun could exert, an influence across great lengths of empty space, but he claimed that the strength and direction of the unknown influence (which he called a 'force') should be judged by the 'amount of motion' that it created. Unfortunately he never explained his reasons for this opinion. We are left to guess.

I urge that 'amount of motion' should be an early study of dynamics. It can be taught by using a collection of trolleys running on almost frictionless wheels. The amount of substance in each should be adjusted till any two trolleys starting at rest recoil at equal speeds when a spring is released between them. Then we know that they have equal inertia (equal mass). Perhaps each one might amount to one kilogram. The trolleys need not be alike in design or materials, but just in mass.