In general, mass is the amount of matter in a body. Mass can be defined more precisely in terms of how difficult it is to change a body's state of motion or how great is the body's gravitational effect on other objects. The first of these is called inertial mass (see inertia) and is given by the factor m in Newton's second law F = ma (see Newton's laws of motion). The second is called gravitational mass and is the mass corresponding to an object's weight in a local gravitational field – the m in F = mg for an object on or near the Earth. According to all experiments, the values for m arising from these two definitions are identical.
Einstein's mass-energy relationship shows that mass and energy are interchangeable. For this reason, the law of conservation of mass in classical physics has been extended to become the law of conservation of mass and energy.
The SI unit of mass is the kilogram.
The difference between mass and weight
The downward pull of the potatoes depends on two things: the amount of matter contained in the potatoes (their mass) and the gravitational pull of the Earth. Unless bits are chopped of the potatoes their mass stays the same, but the Earth's gravitational pull varies depending on location. For example, a stone dropped down a well will have an increased velocity of about 9.8 m/s for every second it falls. But if the same stone were to fall from the same height as a satellite in orbit it's increase in velocity per second would be much less. This is because the stone would be further away from the Earth and thus less strongly affected by its gravitational field. Using a spring instrument, our bag of potatoes would weigh slightly less on a mountain top than at sea level.
But the mass of an object doesn't depend on location. A block of lead, for instance, is composed of a particular number of atoms. Each lead atom (of the commonest isotope) has 82 protons, 126 neutrons, and 82 electrons, and has a constant mass. It doesn't matter if the block of lead is down a well, up a mountain, or on the surface of the Moon, provided nothing has been chopped off it or stuck on it, it has the same mass.
Because of this, mass has to be measured using an instrument that will give the same answer wherever it is used. The beam balance is an example of such an instrument. The object of unknown mass is placed on the left hand balance pan. The pan drops and the beam tilts because their is nothing on the other pan to balance it. Metal "weights" (badly named – they should be called masses!) are placed on the other pan until a balance is achieved – mass is balanced against mass. When the balance is taken to a place where the gravitational field is less intense, there is less pull on the object, but also equally less pull on the "weights" in the other pan, and the same result is obtained.
The chemical balance is used for measuring masses to four decimal places. The weights must be treated with great care. If they become chipped, they lose some mass. If they are picked up with the fingers, grease from the hands attracts dirt and there is an increase in mass. They should always be handled with tweezers. This is one of the reasons why some of the more accurate balances are handled by remote control.
Related categories• CLASSICAL MECHANICS
• GRAVITATIONAL PHYSICS
• PROPERTIES OF MATTER
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