"proper acceleration" meaning in English

{
  "forms": [
    {
      "form": "proper accelerations",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "proper acceleration (plural proper accelerations)",
      "name": "en-noun"
    }
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  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
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          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
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        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Acceleration",
          "orig": "en:Acceleration",
          "parents": [
            "Physics",
            "Sciences",
            "All topics",
            "Fundamental"
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          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Physics",
          "orig": "en:Physics",
          "parents": [
            "Sciences",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "The acceleration felt by an object, as opposed to the coordinate acceleration. For example the acceleration of an object at the surface of the Earth is 9.81 m/s² upwards, but has no coordinate acceleration. The proper acceleration of an object is Lorentz invariant."
      ],
      "id": "en-proper_acceleration-en-noun-w0-waghu",
      "links": [
        [
          "physics",
          "physics"
        ]
      ],
      "raw_glosses": [
        "(physics) The acceleration felt by an object, as opposed to the coordinate acceleration. For example the acceleration of an object at the surface of the Earth is 9.81 m/s² upwards, but has no coordinate acceleration. The proper acceleration of an object is Lorentz invariant."
      ],
      "topics": [
        "natural-sciences",
        "physical-sciences",
        "physics"
      ]
    }
  ],
  "word": "proper acceleration"
}

{
  "forms": [
    {
      "form": "proper accelerations",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "proper acceleration (plural proper accelerations)",
      "name": "en-noun"
    }
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  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
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        "English countable nouns",
        "English entries with incorrect language header",
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        "English nouns",
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        "en:Acceleration",
        "en:Physics"
      ],
      "glosses": [
        "The acceleration felt by an object, as opposed to the coordinate acceleration. For example the acceleration of an object at the surface of the Earth is 9.81 m/s² upwards, but has no coordinate acceleration. The proper acceleration of an object is Lorentz invariant."
      ],
      "links": [
        [
          "physics",
          "physics"
        ]
      ],
      "raw_glosses": [
        "(physics) The acceleration felt by an object, as opposed to the coordinate acceleration. For example the acceleration of an object at the surface of the Earth is 9.81 m/s² upwards, but has no coordinate acceleration. The proper acceleration of an object is Lorentz invariant."
      ],
      "topics": [
        "natural-sciences",
        "physical-sciences",
        "physics"
      ]
    }
  ],
  "word": "proper acceleration"
}