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Inertial vs gravitational mass
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Their equality is the equivalence principle.
Inertial mass measures resistance to acceleration; gravitational mass measures the response to gravity. Their equality (tested to 10⁻¹⁵) is why all bodies fall equally.
Eötvös and torsion balance experiments.
This equality is the starting point for general relativity.
In more detail
Inertial mass measures resistance to acceleration, while gravitational mass measures the strength of gravitational attraction. Experiments show that the two are equal to high precision.
Real-world uses
- Equal inertial and gravitational mass is the basis of general relativity.
- Precision tests such as torsion balance experiments confirm their equality.
- Experiments comparing the time for different materials to fall in a vacuum test whether inertial and gravitational mass are equal.
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Frequently asked questions
Why are they equal?
No one has found a reason, but this equality is central to the equivalence principle.
How precisely are they equal?
Tests agree to parts in 10¹³ or better.
Why does equality of the two masses matter?
It explains why all objects fall with the same acceleration, which is the basis of general relativity.
Related topics in Gravitation & Orbits
Dark matter evidence · Newton's shell theorem · Gravitational self-energy · Equivalence principle · Orbital precession · Galaxy and cosmic structure