|इनार्सियाको[[इनार्सिय्ल सन्दर्भमाफ्रेम रिफरेन्स]]मा, कुनै वस्तुमा बाहिरी [[बल]] नलगाएसम्म उक्त वस्तु स्थिर छ भने स्थिर अवस्थामै रहिरहन्छ र चाल अवस्थामा छ भने समान गतिले अगाडि गइरहन्छ । .<ref name=first-law-shaums>{{Cite book| last = Browne| first =Michael E.| title =Schaum's outline of theory and problems of physics for engineering and science| publisher = McGraw-Hill Companies| date =July 1999| format =Series: Schaum's Outline Series| pages =58| url =https://books.google.com/?id=5gURYN4vFx4C&pg=PA58&dq=newton's+first+law+of+motion&q=newton's%20first%20law%20of%20motion| isbn =978-0-07-008498-8}}</ref><ref name=first-law-dmmy>{{Cite book| last = Holzner| first = Steven | title =Physics for Dummies| publisher =Wiley, John & Sons, Incorporated| date =December 2005| pages =64| url =https://books.google.com/?id=FrRNO6t51DMC&pg=PA64&dq=Newton's+laws+of+motion&cd=8#v=onepage&q=Newton's%20laws%20of%20motion| isbn =978-0-7645-5433-9}}</ref>
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|<!-- Beware! Both the acceleration and momentum formulations of Newton's second law are valid ''only'' for constant-mass systems. This is covered in the discussion below and in multiple references given. -->
इनार्सियाको[[इनार्सिय्ल सन्दर्भमाफ्रेम रिफरेन्स]]मा, कुनै पनि वस्तुमा समग्र [[बल]] {{math|'''F'''}}को (बलहरुको भेक्टर योगफल भनेको) त्यो वस्तुको [[पिण्ड]] (mass) {{math|''m''}} र वस्तुको [[प्रवेग]] (acceleration) {{math|'''a'''}} को गुणन संग बराबर हुन्छ । <br> {{math|'''F''' {{=}} ''m'''''a'''}}<br> (वस्तुको पिण्ड "m" लाइ चर मानिएको छ – see [[#Newton.27s_second_law|below]].)
