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2.2.1 Statics

Mass, force and weight

Mass
Mass (symbolized ‘m’) is a dimensionless quantity representing the amount of matter in a
particle or object. The standard unit of mass in the International System (SI) is the kilogram (kg),
and the slug in the imperial system.

1 slug = 14.59 kilogram

In physics, mass is a property of a physical body. It is a measure of an object’s resistance to
acceleration (a change in its state of motion) when a force is applied. It also determines the
strength of its mutual gravitational attraction to other bodies.

Force
The physicist uses the word ‘force’ to describe any push or pull. A force is one kind of vector. A
vector is a quantity that has both size and direction.

A force has a certain magnitude or size. Also, a force is always in a certain direction. To
completely describe a force, it is necessary to specify both the size of the push or pull and its
direction.

The units in which force are measured are the pound (lb) in the imperial system and the newton
(N) in the metric system. The newton is named after Sir Isaac Newton, a famous British
physicist who lived in the 17th century.

The relationship between the metric and imperial units is given by the conversion factor:

1 lb = 4.448 N

Weight
A weight is one kind of force. It is defined as the gravitational pull of the earth on a given body.
The direction of this force is toward the geometrical centre of the earth.

Distinction between mass and weight
The physicist very carefully distinguishes between ‘mass’ and ‘weight’. As we have seen, mass
is the quantity of matter, determined by the number of protons and neutrons in the body, and
weight is a measure of the gravitational pull of the earth on this quantity of matter.

It may seem that this is an unimportant distinction. However, there is one important difference.

The mass of an object is the same wherever this object is in the universe. The mass of a stone
is the same if the stone is on the earth, on Mars, in a space ship, or some place in the Milky
Way Galaxy. If the stone is not on the earth but is in a space station orbiting the earth some
distance from the earth’s surface, the weight of this stone is different from its weight on the
earth’s surface. If the stone is on the planet Mars, we speak of its ‘weight on Mars’, the
gravitational pull of Mars on the stone.

Total Training Support Ltd 2-7 Issue 2 – September 2016
© Copyright 2016 Module 2.2 Mechanics