Measurements of mass are made most commonly by using a balance. An unknown object’s mass is compared with the known mass of a check standard to yield its value.
An object’s mass is related to its inertia, which is resistance to acceleration (change of velocity). This article will discuss the concept of mass and the unit kilogram.
Gravitational Force
The force that objects exert on each other due to their mass is called gravitational force. Its magnitude depends upon the masses of the two objects and the distance between them squared (F = G
The value of G is important for understanding weight measurement because an object with a large mass has more gravity than an object with less mass. This greater gravitational force causes the object with more mass to accelerate faster given the same unbalanced force.
In the 1790s, Henry Cavendish used a delicate torsion balance to measure the strength of this attraction between masses and to determine G. His results showed that this universal constant is a true constant, not affected by the composition of the masses or the location; it remains the same throughout the universe.
Inertial Force
Objects that have more mass will resist a change in their state of rest or motion more strongly than objects with less mass. For example, moving a truck will require more force than moving a bike since the truck has more mass. This is because more mass has greater inertia.
One of the most basic laws of physics is the law of universal gravitation, which states that all matter falls at the same rate regardless of the object’s location. This is also why a feather will fall at the same speed as a hammer even though they have different masses.
Until modern times, what we now know as mass was commonly referred to as weight. Ancient goldsmiths used a balance to measure the “heaviness” of gold. Later, a number of different systems were used to determine weight, with 180 grains making up a shekel, 60 of these forming a pound, and 600 of these making up a kilogram.
Force of Gravity
The force of gravity, often referred to as g, is the acceleration that all objects experience due to the distribution of mass within Earth. This force is modified by centrifugal effects, resulting in the gravitational acceleration that we experience on our own planet.
The strength of the gravitational force between two bodies depends on their masses and on the distance between them, according to Newton’s second law. This is the basis of balances for measuring weight in space and in places with no gravity, where a known value for gravity (g) is applied to the measurement.
The SI unit of mass is the kilogram, originally defined as one cubic decimeter of water at its density limit. It was later redefined by removing the reference to this and using the Planck constant as its fixed value. The primary standard kilogram is a platinum-iridium cylinder kept at NIST. Other kilograms are based on this international prototype. The kilogram is also used as the base for many other units of measurement.
Kinetic Energy
In classical mechanics, an object’s kinetic energy depends on its mass and velocity. Its kinetic energy is proportional to its square speed, so it takes four times as much effort to stop it if it doubles its speed, assuming a constant braking force is used.
An object with a large mass and a fast speed has a lot of kinetic energy, as does a high jumper coming back down from the air. In fact, all objects in motion possess kinetic energy.
The formula for kinetic energy is: 1/2mv2. This represents the total energy possessed by an object or particle due to its translation, rotation, vibration, electron translation and spin, and nuclear spin. Since an object’s speed is a vector, its kinetic energy also depends on the reference frame in which it is measured. However, the magnitude of an object’s kinetic energy is a scalar quantity, so it doesn’t depend on its direction. The only exception is when the object reaches close to the speed of light, when Einstein’s special theory of relativity must be employed.