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What is a gravitational field and how its strength be measured

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A gravitational field is the field generated by a massive body, that extends into the entire space. Every object with mass m experiences a force F when immersed in a gravitational field. The intensity of the force is equal to

F= (GM)/(r^2) m
where
G=6.67 \cdot 10^(-11) m^3 Kg^(-1) s^(-2) is the gravitational constant, M is the mass of the source of the field (e.g. the mass of a planet), and r is the distance between the object and the source of the field. The force is always attractive.

A possible way to measure the intensity of a gravitational field is by measuring the acceleration a of the object immersed in this field. In fact, for Newton's second law we have:

F=ma
but since

F= (GM)/(r^2) m
we can write

a = (GM)/(r^2)
Therefore, by measuring the acceleration of the object, we also measure the intensity of the field.

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