A weighing scale measures mass by measuring the gravitational force exerted by the object we measure. Because the magnitude of the force depends on the strength of the gravitational acceleration, the result can vary depending on the location. It is the reason the scale needs to be calibrated after it is moved to another location.
On the other hand, a balance measures mass by comparing the object’s mass to a previously known mass. Because both masses are affected by the same gravitational acceleration, a balance is not affected by the change in gravitational acceleration.
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Earth pulls all objects downward by the same gravitational acceleration of 9.8 m/s². But the force of gravity exerted by Earth on an object still depends on its mass. An object with a greater mass has a greater force of gravity (also called weight).
Flat-Earthers claim that if greater mass means a greater force of gravity, a bowling ball in a vacuum should fall faster than a feather. In reality, they confuse acceleration with force. Both fall at the same speed because the gravitational acceleration is the same on both, but their forces of gravity are different.
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At a lower altitude, the air is denser, and objects weigh more. On the other hand, at a higher altitude, the air is less dense, and objects weigh less.
Flat-Earthers deny gravity and invented the “explanation” that an object falls only because it is denser than the air surrounding it. But if it were true, objects should weigh more at a higher altitude where the air is less dense. But they weigh less instead, disproving flat-Earthers’ density theory.
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Archimedes’ principle states that the upward buoyant force exerted on a body immersed in a fluid is equal to the weight of the fluid that the body displaces. Today we usually use B = -ρgV to calculate the buoyant force, where ρ is the fluid’s density, g is the gravitational acceleration, and V is the volume of the displaced fluid.
Archimedes discovered buoyancy earlier than Newton discovered gravity, and flat-Earthers dispute the presence of g in the buoyancy formula. In reality, buoyancy depends on the weight of the fluid, and the distinction between weight and mass only occurred after Newton. Archimedes’ principle still applies, only that we now have a better understanding of what weight is.
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The acceleration of free-falling objects on Earth varies. It depends on various factors. The figure of 9.8 m/s² is just a nominal value to use if the actual local value is unknown or unimportant. These variations can only be explained using the law of gravity and that the Earth is a rotating sphere.
Flat-Earthers claim that science tells us that Earth’s gravitational acceleration is always 9.8 m/s² everywhere on Earth, and they use it as “evidence” Earth is flat and stationary. But, in reality, Earth’s gravitational acceleration does indeed vary in different locations.
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The weight of an object is the force exerted on the object by gravity. The gravitational force exerted by the Earth on an object is what we feel as the ‘weight’ of the object.
Many flat-Earthers fail to understand that the gravitational force is just another name for ‘weight’. Much misinformation in flat Earth communities arises from this misunderstanding.
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The motion of a specific location on Earth due to Earth’s rotation is a uniform circular motion. We can separate it into a uniform linear motion and a centripetal acceleration toward Earth’s center. From the reference frame of an observer on Earth, the latter results in a constant centrifugal acceleration, which is felt simultaneously with Earth’s gravitational acceleration.
We often use the uniform linear motion of everyday objects to explain the effect (or the lack of thereof) of uniform motion of Earth’s rotation. Flat-Earthers incorrectly reject it just because “it is not a circular motion unlike Earth’s rotation” In reality, a uniform linear motion is one component of a uniform circular motion. And these explanations are correct and still apply.
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A density tower is formed by layering immiscible liquids of different densities on top of each other. It is possible because Earth’s gravitational acceleration pulls denser fluid particles stronger and displace less dense ones upward.
Flat-Earthers claim that the possibility to have density towers disproves gravity. In reality, a density tower cannot form without the presence of acceleration, like Earth’s gravitational acceleration.
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