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Newton's laws and inertial frames
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The laws hold in inertial frames; failing that, add pseudo forces.
Newton's laws hold exactly only in inertial frames. Earth is nearly inertial for short experiments. To use Newton's laws in rotating or accelerating frames, add fictitious forces.
A Foucault pendulum's rotating swing plane reveals Earth's rotation.
The laws don't claim that the frame is special, only that it is inertial.
In more detail
Newton's laws hold in inertial frames, and in other frames they require fictitious forces. Choosing an inertial frame is usually the simplest way to apply them.
Real-world uses
- Spacecraft navigation uses inertial frames fixed to the stars.
- Accelerating cars require fictitious forces in the car's own frame.
- Space missions model their motion in an inertial frame, and add pseudo forces only when they work in a rotating frame such as a spinning station.
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Frequently asked questions
Is the lab floor an inertial frame?
Approximately, for most experiments, since Earth's rotation and motion are small effects.
How do I know if a frame is inertial?
If a free object moves in a straight line at constant speed without any real force, the frame is inertial.
Is the Earth an inertial frame?
Only approximately. Its rotation and orbit introduce small effects, so it is a good approximation for everyday experiments.
Related topics in Newton's Laws & Forces
Motion of center of mass · Normal force on curves · Constraint forces · Dimensions and limits of Newtonian mechanics