# Newton's First Law of Motion is Wrong (Proven Wrong in This Thread)

## Main Question or Discussion Point

First law: When viewed in an inertial reference frame, an object either remains at rest or moves at a constant velocity, unless acted upon by an external force.

Here is an experiment to prove this First Law wrong.
Consider two balls rolling at a constant velocity directly toward each other on a frictionless plane. Ball A is heading due east at 5 m/s and Ball B is heading due west at 5 m/s. The balls engage in an inelastic collision. The balls will both come to rest. This can be proven experimentally.

Please note that Newton's Second Law describes Force = Mass * Acceleration. Neither ball has an acceleration, meaning that neither ball has any force associated with it. So despite not being acted upon by any FORCE, during the collision, the balls change velocity from 5 meters/second to zero. This simple experiment clearly shows that an object will not necessarily stay in motion at a constant velocity despite not being acted upon by an external force.

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Doc Al
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Please note that Newton's Second Law describes Force = Mass * Acceleration. Neither ball has an acceleration, meaning that neither ball has any force associated with it. So despite not being acted upon by any FORCE, during the collision, the balls change velocity from 5 meters/second to zero. This simple experiment clearly shows that an object will not necessarily stay in motion at a constant velocity despite not being acted upon by an external force.
This is silly. When the balls collide they exert forces on each other!

This is silly. When the balls collide they exert forces on each other!
Please note Newton's Second Law of Motion:

Second law: The vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object; thus, F = ma.

Please note that the acceleration of both balls are equal to 0. Please note that mass*0=0. Consequently, neither ball produces a force (or rather, produces a force equal to 0).

Doc Al
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Please note that the acceleration of both balls are equal to 0.
Only before they collide! Once they collide, they certainly have a non-zero acceleration. You indicated such yourself!

Nugatory
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Please note Newton's Second Law of Motion:

Second law: The vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object; thus, F = ma.

Please note that the acceleration of both balls are equal to 0. Please note that mass*0=0. Consequently, neither ball produces a force (or rather, produces a force equal to 0).
In the collision, both balls change their speed from five meters/second to zero meters per second. That's an acceleration, and you can even calculate its average value throughout the collision: If the time between when the balls first touch andwhen they stop moving is $\Delta{t}$ seconds, the average magnitude of the acceleration will be $\frac{\Delta{v}}{\Delta{t}} = \frac{5}{\Delta{t}} m/sec^2$.

Non-zero $a$, non-zero $m$, gotta be a non-zero $F$.

Dale
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