Weight and Mass

n everyday speech, the words weight and mass are used to mean the same thing. To scientists and engineers, however, they have different meanings. Mass is the amount of matter an object is made of. Its weight is the downward force acting on it due to gravity. Mass is a number, but weight is a force. Mass is constant, but weight depends on the strength of gravity acting on the mass.

The weight and mass of an object are linked in the following equation: weight = mass x g

in which g is the acceleration the object would experience if it fell. At Earth’s
surface, this is 32 feet per second per sec­ond (9.75 meters per second per second).

For most of human history, an object’s weight meant its weight on Earth’s surface. As gravity is more or less the same everywhere on Earth, it did not matter if weight and mass were mixed up. It matters today, because spacecraft can be sent into Earth orbit and away to the Moon and planets, where the force of gravity acting on them may be quite different from the strength of Earth’s gravity.

Mass

If an object is given a push, it acceler­ates. The size of acceleration depends on the mass of the object. If a small mass and a large mass are pushed by the same force, the small mass accelerates faster. This is because the large mass offers more resistance to the force than the small mass.

The tendency of a mass to resist a change in its motion is called inertia. The bigger the mass, the greater the inertia. This is impor­tant when calculating how aircraft and spacecraft will accelerate when their engines produce a cer­tain amount of thrust.