**NEWTON’S 2**^{nd} LAW

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*F* = *m* *a*

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*F* is the “force”

*m*** **** is the “inertia” of “mass”**

*a*** **** is the “acceleration”**

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**The
acceleration can be measured in terms of lengths and times (change in speed per
unit time) such as (meters/second)/second**

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**Force has **__no__ independent meaning outside this
equation.

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**Masses are determined relative to each other in
collisions, hence their absolute value involves an arbitrary constant.**

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**Thus the equation ***F* = *m* *a*

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# Could equally well have been written by Newton
as

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** (***Fc*^{2})
= (*mc*^{2 }) *a*

**or**

* f =
M a*

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**Thus ***E** ***= ***mc*^{2} is
like converting an absolute temperature from Fahrenheit to Celsius degrees –

**all**** are measures of aspects of the energy of a system.**

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