The gravity of Earth varies on the different location on Earth. The variation is caused by the difference in latitude, altitude or depth, and local geology & topography. In most cases, the centrifugal acceleration from Earth’s rotation is also already accounted for in the value of Earth’s gravitational acceleration.
Flat-Earthers claim that according to science, the Earth’s gravitational acceleration is always constant at 9.8 m/s² everywhere on Earth. And they are using the claim to prove Earth is not a rotating sphere. In reality, the magnitude of Earth’s gravitational acceleration varies on the different location on Earth.
The value of 9.8 m/s² is only an approximation. It is a nice, round number that is very close to the actual value everywhere on Earth, and close enough for most practical applications. The real value does differ by about 0.5%. An object at the poles will weight approximately 0.5% more than at the Equator.
The various factors that determine the magnitude of Earth’s gravity:
- Latitude. The equator is farther away from the center of the Earth than the poles. The equator is also getting the highest centrifugal acceleration due to Earth’s rotation.
- Altitude/depth. At a higher altitude, the gravity is lower due to the increased distance from the center of the Earth. Deeper into the Earth, the gravity is also lower because the mass above us will pull us upward instead of toward the center of the Earth.
- Local topography & geology. If the part of the Earth close to the observer has higher density, it will exert higher gravity to the observer.
Taking a scale to another location will no longer result in the correct measurement, and will require recalibration.
Flat-Earthers correctly hypothesized that if Earth is rotating, then it will counteract Earth’s gravitational acceleration more at the equator than closer to the poles. They did not get the fact right, however. Earth’s gravity does vary according to latitude and it is consistent with the spherical Earth model.