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Why bother with Newton 1 (solution)

Page history last edited by Joe Redish 4 years, 6 months ago

4.1.2.P17

 

Newton's first law states that an object will move with a constant velocity if nothing acts on it. This seems to contradict our everyday experience that all moving objects come to rest unless something acts on it to keep it going. Does our everyday experience contradict one of Newton's Laws? If it does not, explain the apparent contradiction. If it does, explain why we bother to teach Newton's first law anyway. Note: This is an essay question. Your answer will be judged not solely on its correctness, but for its depth, coherence, and clarity.

 

Our everyday experience actually contains evidence that contradicts itself. Yes, objects eventually seem to come to rest if nothing acts on them. But when objects are moving -- especially moving fast or with a large mass, like a 100 mi/hr fast ball or a high speed train -- we have the intuition that we don't want to mess with them! They seem to want to keep moving. Saying "they'll eventually slow down and stop" seems a cop out and doesn't give us any hints for how to deal with the fast moving object.

 

What Newton 1 does is it tells us where to look for something acting on the object. By choosing "motion with a constant velocity" as the "natural" motion, that is, the motion that doesn't need any explanation, it tells us where to look for "something acting to change the motion." It leads us to conjecture the existence of "invisible forces" -- actors that we don't see obviously, such as the earth pulling down on objects that don't touch it, and friction between two surfaces rubbing together. What happens is that we make such a conjecture and then study what the properties of these invisible forces would be. What we find is we only have to conjecture a small number of such forces and they have consistent properties that can be measured (like the gravitational field and the friction coefficient). This then allows us to explain essentially all motions -- including unifying motions on the earth and of planets and moons in the sky!

 

(Other answers are possible. What we are looking for is an understanding of the physics and a discussion of the reconciliation between everyday experience and Newton 1.)

 

Joe Redish 10/8/02

 

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