Wednesday, July 18, 2012

Shay 11 and Newton's 2nd Law

Our group had a wonderful experience today.  We rode a Shay Steam Locomotive to the top of Bald Knob, elevation 4730 feet.  One the more interesting physics concepts that jumped out at me was the instances when the train would begin its motion and come to a rest. 

With some scrutiny, you could recognize the coupling between the cars and hear a "clank, clank, clank" when starting and stopping.  This brought to my mind the essence of the great. Isaac Newton and his 2nd Law of Motion.  We are most certainly familiar with the notion that F=ma.  Yet, I think it is imporant to remind ourselves that this not exactly what Newton said in his masterpiece Principia.  More precisely he said that the sum of the impulses acting on a body is equal to the net change in the bodies momentum. We of course quantify this as F delta t = delta p.  This change in momentum for a train of course must huge, as mass is being increased with each additional car and with a change in the velocity.  The key to this is really a term we use often and is in our equation.  It begins with "t" and rhymes with "ime"; yes, it is time, time, time. 

A force applied during a greater period of time will equal a greater change in momentum and this is what is needed to start or or stop.  So, the next time you hear a train go "clank, clank, clank" I want you to think Newton and his 2nd Law of Motion.

Here is a sample. If a train with a weight of 800 tons wants to achieve a velocity of 10 km/hr, how much time must be allowed if the maximum tractive force of 16000 Newtons?  Mind your conversion and remember what weight is.







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