Cracking the SAT Physics Subject Test

WEIGHT

For this test, remember that although they are used interchangeably in everyday life,
mass and weight are not the same thing; there is a clear distinction between them in
physics. The weight of an object is the gravitational force exerted on it by Earth (or
by whatever planet on which it happens to be). Mass, in contrast, is an intrinsic
property of an object that measures its inertia: An object’s mass does not change
with location. If you put a baseball in a rocket and send it to the moon, its weight on
the moon would be less than its weight on Earth (because the moon’s gravitational
pull is weaker than Earth’s due to its much smaller mass), but its mass would be the
same.

Since weight is a force, we can use Fnet = ma to compute it. What acceleration

would gravitational force impose on an object? The gravitational acceleration, of
course! Therefore, setting a = g, the equation Fnet = ma becomes

Fw = mg

This is the equation for the weight of an object of mass m (weight is often
symbolized just by w, rather than Fw). Notice that mass and weight are proportional

but not identical. Furthermore, mass is measured in kilograms, while weight is
measured in newtons.

G or g?

Remember that G is a

universal constant equal to

6.67 × 10−11 N·m^2 /kg^2.

g depends upon what

planet or moon an object

is on. Near the surface,

mg = where M and

R are the mass and radius

of the planet or moon.

Cancelling m, we see that

g =.