# What is the proof of the law of inertia?

**URL:** <https://boards.straightdope.com/t/what-is-the-proof-of-the-law-of-inertia/159352>\
**Category:** Factual Questions\
**Created:** [March 5, 2003, 10:08pm UTC](https://boards.straightdope.com/t/what-is-the-proof-of-the-law-of-inertia/159352 "2003-03-05T22:08:51Z")\
**Posts on this page:** 3\
**Page:** 2

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**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\
**Post date:** [March 6, 2003, 7:31pm UTC](https://boards.straightdope.com/t/what-is-the-proof-of-the-law-of-inertia/159352/21 "2003-03-06T19:31:14Z")

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> [@](#):
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> \*Originally posted by FranticMad \*  
> \*\*The fact that we can send satellites into high orbit, and they stay there for a very long time, is a pretty convincing demonstration of inertia. \*\*

This is actually not a very good example for Newton’s First Law (a.k.a the Law of Inertia). Newton’s First Law may be stated as: “An object at rest tends to stay at rest and an object in motion tends to stay in motion with constant velocity unless acted upon by a net external force.”

An orbiting satellite (in a circular orbit) has a constant net external force acting on it (the gravitational force of attraction on the satellite by the Earth), and the satellite is under a constant acceleration toward the center of the Earth. While the satellite’s speed may be constant, the velocity is not. (The direction of travel is constantly changing).

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**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\
**Post date:** [March 6, 2003, 8:50pm UTC](https://boards.straightdope.com/t/what-is-the-proof-of-the-law-of-inertia/159352/22 "2003-03-06T20:50:07Z")

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> [@](#):
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> \*Originally posted by robby \*  
> \*\*One example is the “Law of Conservation of Mass.” This law was experimentally invalidated with the advent of nuclear physics. \*\*

It is unfortunate in the extreme, but nonetheless I feel I must say this; _universally, mass is a conserved quantity._

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**Author:** ![asterion](https://avatars.discourse-cdn.com/v4/letter/a/4da419/32.png) [@asterion](https://boards.straightdope.com/u/asterion)\
**Post date:** [March 6, 2003, 11:46pm UTC](https://boards.straightdope.com/t/what-is-the-proof-of-the-law-of-inertia/159352/23 "2003-03-06T23:46:02Z")

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> [@](#):
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> \*Originally posted by GTPhD1996 \*  
> \*\*Pravnik
> 
> The definition of acceleration is the rate of change of velocity. So if something is accelerating, it’s velocity is changing.
> 
> GTPhD1996 \*\*

True, but velocity is a vector. Acceleration is a derivative of velocity. You can change a vector by changing its direction and leaving its speed intact.

Just pointing out that no one has brought up angular acceleration and velocity yet.

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