• # Weight

Posted by on April 11, 2023 at 4:26 PM

Weight is the force of gravity acting on an object. The unit of measurement for weight is that of force, which in the International System of Units (SI) is the newton [N].

In many real-world situations, the act of weighing may produce a result that differs from the ideal value provided by the definition used. This is usually referred to as the apparent weight of the object. A common example of this is the effect of buoyancy when an object is immersed in a fluid the displacement of the fluid will cause an upward force on the object, making it appear lighter when weighed on a scale.

The main characteristic of weight is that it is directly proportional to mass and varies only when mass varies, because gravity acceleration is constant. Consequently the Earth attracts every body exerting a different force because different are the masses of the bodies on which it exerts it, but in such a way that the acceleration impressed to each body has always the same value. Moreover, gravity force is always directed vertically downwards. Imagining to look at the Earth from the space we will say that the weight is directed towards the center of the planet and, since the force lies on the direction given by the radius, it will always be perpendicular to the surface in every point.

To be precise, the constant g is not really a physical constant. Actually its value changes according to the altitude and more precisely to the distance of the body from the center of the Earth. In the lessons dedicated to universal gravitation we will see in particular that acceleration of gravity decreases with increasing distance from the center of the Earth, therefore a climber on the top of Everest is subject to a lower acceleration and therefore also a lower weight than the one he perceives when he is at sea level. In any case, weight is greater at the poles while it is less at the equator, and this is due to the fact that the Earth is not perfectly spherical: being more swollen at the equator, on this parallel the distance from the center is greater and therefore the weight is less with the same mass.

But the question is: how much does the acceleration of gravity change, and therefore how much does the weight change, with the same mass and varying the distance from the center of the Earth? In fact we are dealing with percentage variations less than 1% so that g can be treated in good approximation as a constant, provided however to remain close to Earth surface.

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