Mass Versus Weight

Mass Versus Weight

In everyday usage, the mass of an object is often referred to as its weight though these are in fact different concepts and quantities. In scientific contexts, mass refers loosely to the amount of "matter" in an object (though "matter" may be difficult to define), whereas weight refers to the force experienced by an object due to gravity. In other words, an object with a mass of 1.0 kilograms will weigh 9.8 newtons (newton is the unit of force, while kilogram is the unit of mass) on Earth (its mass multiplied by the gravitational field strength). Its weight will be less on Mars (where gravity is weaker), more on Saturn, and negligible in space when far from any significant source of gravity, but it will always have the same mass.

Objects on the surface of the Earth have weight, although sometimes this weight is difficult to measure. An example is a small object floating in a pool of water, or even a dish of water, which does not appear to have weight since buoyed by the water, but is found to have its usual weight when it is added to water in a container which is entirely supported and weighed on a scale. Thus, the "weightless object" floating in water actually transfers its weight to the bottom of the container (where the pressure increases). Similarly, a balloon has mass but may appear to have no weight or even negative weight, due to buoyancy in air. However, in the case of buoyancy, the weight of the balloon and the gas inside it has merely been transferred to a large area of the Earth's surface (in fact the entire surface, eventually), making the weight difficult to measure. The weight of a flying airplane is similarly distributed to the ground, but does not disappear. If the airplane is in level flight, the same weight-force is distributed to the surface of the Earth as when the plane was on the runway, but spread over a larger area.

A better scientific definition of mass describes it as having inertia, the resistance of an object to being accelerated when acted on by an external force. Gravitational "weight" is the force created when a mass is acted upon by a gravitational field and the object is not allowed to free-fall, but is supported or retarded by a mechanical force, such as the surface of a planet. Such a force confers weight. Such weights can be added to by the weight created from any kind of mechanical force.

For example, in the illustrative photograph, the girl’s weight (force due to gravity and the inertial force resisting circular acceleration) is being supported by the swing seat and swing set. If one stands behind her at the bottom of her arc and abruptly stops her, the "bump" or stopping-force one experiences is due to acting against her inertia, and would be the same force even if gravity were not present. Such forces also confer a type of weight, which could be measured.

While the weight of a mass is a function of the strength of gravity, the mass of an object is constant for any given observer, so long as no energy or matter is added to the object. Accordingly, for an astronaut on a spacewalk in orbit (a free-fall), no effort is required to hold a communications satellite in front of him; it is "weightless". However, since objects in orbit retain their mass and inertia, an astronaut must exert ten times as much force to accelerate a 10‑ton satellite at the same rate as one with a mass of only 1 ton.

On Earth, a swing set can demonstrate this relationship of force, mass, and acceleration without being appreciably influenced by weight (downward force). If one were to stand behind a large adult sitting stationary in a swing and give him a strong push, the adult would accelerate relatively slowly and swing only a limited distance forwards before beginning to swing backwards. Exerting that same effort while pushing on the small girl would produce much greater acceleration.