Why is an an astronaut's weight different on different planets?

Answer 1

Weight, a commonly misunderstood term, is the force of gravity on a mass.

Mass is constant, but weight can vary depending on the gravitational field.

Gravity is a force that is exerted on an object by another object.

The formula for weight is:

G x m1 x m2 / r2

where G is the "gravitational constant",

m1 is the mass of one object and m2 is the mass of the other object.

The distance between the objects is r.

The gravity of a planet acts as if all the mass is at the center of the planet.

So, for an astronaut on the planet surface, r in the equation is the radius of the planet.

The weight of an astronaut is different on different planets because the masses of the planets and their diameters are different from the Earth, making the force of gravity stronger or weaker, thus altering the weight.

In fact the "surface gravity" on Mars and Mercury is almost exactly equal. That's because Mars has a bigger mass, but also a bigger diameter. In this case, these two factors cancel out almost exactly.

Incidentally, an astronaut couldn't actually stand on the surface of one of the "gas giant" planets because the "surfaces" are gas.

More details: the rotation speed of the planet affects the effective gravitational force you feel, because of centrifugal effects.

This is particularly important in the case of large fast spinning planets like Jupiter and Saturn.