What is the measure of the force of gravity on an object?

Weight

Force is the vector product of mass and acceleration: F = ma. Weight is a special case of that formula, where you substitute the acceleration of gravity, g, for a. We can therefore write: W = mg.¹

For example, if an object has a mass of 10 slugs², its weight near the surface of the Earth is 10 x 32.2 (ft/s²) = 322 pounds (pound-force). If an object has a mass of 10 kilograms, its weight near the surface of the Earth is 10 x 9.8 (m/s²) = 98 newtons.

You can measure the force of gravity on an object (i.e., its weight) by putting it on a scale.

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Strictly speaking, the Force due to gravity is always measured between two objects. The acceleration due to gravity ('a') in the above discussion is a special case of acceleration that occurs due to the force of gravity present at sea level on Planet Earth.

We can calculate the gravitational force between any two objects using the following forumla:

F_g = G (m₁ x m₂) / r²

Where:

F_g - is the force due to gravity

G - is the unviersal gravitational constant

m₁ - is the mass of the first object

m₂ - is the mass of the second object

r - is the distance between the centres of the two objects

When using the SI units, G is 6.67 x 10^-11 N / m² / Kg²

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^{1. Hyper-correctly, we should say to substitute -g for a in the equation, because many physicists think of g as a negative quantity, inasmuch as it's a vector in the downward direction; that is, it is directed toward the center of the Earth. When dealing with weight and the acceleration due to gravity, one must be careful to be consistent in assigning the signs.}

^{2. Whenever possible, use the slug, not the pound-mass, in making calculations of mass. The whole pound-mass/pound-force thing is infuriatingly confusing for students. (And some teachers, too.)}

First answer by Redbeard. Last edit by Konacq. Contributor trust: 34 [recommend contributor]. Question popularity: 53 [recommend question]