diff --git a/physics/energy_mass_equivalence.py b/physics/energy_mass_equivalence.py
new file mode 100644
index 000000000000..24e97b6af7a4
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+++ b/physics/energy_mass_equivalence.py
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+"""
+Calculates the energy equivalent (in J) of a given mass (in kg).
+
+- In physics, mass–energy equivalence is the relationship between mass and energy in a system's rest frame.
+- The two quantities differ only by a multiplicative constant and the units of measurement.
+- The principle is described by the physicist Albert Einstein's formula: E = mc².
+
+Source: https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence
+
+"""
+
+
+def energy_eq(mass: float) -> float:
+    """
+    Calculate energy equivalent.
+    The energy equivalent of a mass m is mc², where c is the speed of light.
+
+    >>> energy_eq(0.001)
+    89875517873681.77
+
+    """
+    if mass < 0:
+        raise ValueError("The mass of a body cannot be negative")
+    c = 2.99792458e8
+    return mass * c**2
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()