Section 3.1 Force
What is force? In simple terms, a force is a push or a pull. More precisely, a force is a vector quantity which tends to produce an acceleration of a body in the direction of its application. This means that if you want a stationary object to move, you must apply a force to it to get it going. If you want to keep it moving, you need to continuously apply a force to it to overcome friction, which is itself a force. Forces can exist without motion too – when an object is stationary it’s usually because there are many forces acting on it which cancel each other out, leaving no net force acting on the object. Forces always occur in pairs. A push on an object is always accompanied by an equal and opposite force acting on the pusher.
In the English system of units, force is measured in pounds, tons, or long tons. In the SI system, force is measured in newtons (N) or kilonewtons (kN). For estimation purposes, a newton is about a quarter of a pound.
One important force which we experience every day is weight. Weight is the force of gravity - the attraction between an object and the earth. The weight force exerted on an object is proportional to the mass of the object according to Newton’s second law, which states:
\begin{equation*}
W = mg
\end{equation*}
where W is the weight, m is the mass, and g is the gravitational constant.
The constant g depends on where you are: on earth the constant is greater than it is on the moon, so you weigh less on the moon than you do on earth. In outer space, the constant is zero, making you weightless there. Be careful not to confuse weight with mass. The mass of an object will not change when it moves to another location, but its weight may. One reason why people often confuse weight with mass is because in the English system and on the surface of the earth, the gravitational constant g is equal to 1, and so one pound of mass has a weight of one pound of force. Move to the moon, and the pound of mass now weighs about six tenths of a pound. On earth in the SI system, g is approximately equal to 9.81 \(\textrm{m/s}^2\text{.}\)
