High Quality Content by WIKIPEDIA articles! In science and engineering, the weight of an object is usually taken to be the force on the object due to gravity. Its magnitude (a scalar quantity), often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus: W = mg. The term weight and mass are often confused with each other in everyday discourse but they are distinct quantities. The unit of measurement for weight is that of force, which in the International System of Units (SI) is the newton. For example, an object with a mass of one kilogram has a weight of about 9.8 newtons on the surface of the Earth, about one-sixth as much on the Moon. In this sense of weight, a body can be weightless only if it is far away from any gravitating mass. There is also a rival tradition within Newtonian physics and engineering which sees weight as that which is measured when one uses scales. There the weight is a measure of the magnitude of the reaction force exerted on a body. Typically, in measuring someone`s weight, the person is placed on scales at rest with respect to the earth but the definition can be extended to other states of motion. Thus in a state of free fall, the weight would be zero. In this second sense of weight, terrestrial objects can be weightless. Ignoring air resistance, an apple on its way to meet Newton`s head is weightless.