A body's mass is a measure of its
*inertia*: *i.e.*, its reluctance to deviate from uniform straight-line
motion under the influence of external forces. According to Newton's second law, Eq. (94),
if two objects of differing masses are acted upon by forces of the same magnitude
then the resulting acceleration of the larger mass is less than that of the smaller
mass. In other words, it is more difficult to force the larger mass to deviate from
its preferred state of uniform motion in a straight line. Incidentally, the mass of
a body is an intrinsic property of that body, and, therefore, does not change if the
body is moved to a different place.

Imagine a block of granite resting on the surface of the Earth.
See Fig. 24. The block experiences
a downward force due to the gravitational attraction of the Earth. This
force is of magnitude , where is the mass of the block and is the acceleration
due to gravity at the surface of the Earth. The block transmits this force to the
ground below it, which is supporting it, and, thereby, preventing it from accelerating
downwards. In other words, the block exerts a downward force , of magnitude ,
on the ground immediately beneath it. We usually refer to this force (or the
magnitude of this force) as the *weight* of the block. According to Newton's third
law, the ground below the block exerts an upward reaction force on the block.
This force is also of magnitude . Thus, the net force acting on the block
is
, which accounts for the fact that the block
remains stationary.

Where, you might ask, is the equal and opposite reaction to the force of gravitational attraction exerted by the Earth on the block of granite? It turns out that this reaction is exerted at the centre of the Earth. In other words, the Earth attracts the block of granite, and the block of granite attracts the Earth by an equal amount. However, since the Earth is far more massive than the block, the force exerted by the granite block at the centre of the Earth has no observable consequence.

So far, we have established that the weight of a body is the magnitude of the downward force it exerts on any object which supports it. Thus, , where is the mass of the body and is the local acceleration due to gravity. Since weight is a force, it is measured in newtons. A body's weight is location dependent, and is not, therefore, an intrinsic property of that body. For instance, a body weighing 10N on the surface of the Earth will only weigh about on the surface of Mars, due to the weaker surface gravity of Mars relative to the Earth.

Consider a block of mass resting on the floor of an elevator, as shown in Fig. 25.
Suppose that the elevator is accelerating upwards with acceleration . How does this
acceleration affect the weight of the block?
Of course, the block
experiences a downward force due to gravity. Let be the weight of the block:
by definition, this is the size of the downward force exerted by the block on the
floor of the elevator. From Newton's third law, the floor of the elevator exerts an upward
reaction force of magnitude on the block. Let us apply Newton's
second law, Eq. (94), to the motion of the block. The mass of the block is ,
and its upward acceleration is . Furthermore, the block is subject to two forces:
a downward force due to gravity, and an upward reaction force . Hence,

(95) |

(96) |

Suppose that the downward acceleration of the elevator matches the acceleration
due to gravity: *i.e.*, . In this case, . In other words, the
block becomes weightless! This is the principle behind the so-called
``Vomit Comet'' used by NASA's Johnson Space Centre to train prospective astronauts in
the effects of weightlessness. The ``Vomit Comet'' is actually a KC-135 (a predecessor
of the Boeing 707 which is typically used for refueling military aircraft). The plane
typically ascends to 30,000ft and then accelerates downwards at (*i.e.*,
drops like a stone) for about 20s, allowing its passengers to feel the effects
of weightlessness during this period. All of the weightless scenes
in the film *Apollo 11* were shot in this manner.

Suppose, finally, that the downward acceleration of the elevator exceeds
the acceleration due to gravity: *i.e.*, . In this case, the block acquires
a negative weight! What actually happens is that the block flies off the
floor of the elevator and slams into the ceiling: when things have settled down, the
block exerts an upward force (negative weight) on the ceiling of the elevator.