Statics is that branch of Mechanics which is concerned with the conditions under which bodies remain at rest relative to their surroundings. In such circumstances bodies are said to be in a state of equilibrium. It is assumed that they are acted upon by ‘forces’ which balance one another. The primary conceptions of Statics are forces and the bodies upon which they act. In Dynamics, force is denned as that which changes or tends to change the state of motion of a body, but in Statics we are not concerned with motion save in so far as a force would cause motion if unbalanced by another force. We get our ideas of force from the ability which we ourselves possess to cause, or to resist, the motion of our own bodies or of other bodies. We are conscious of measurable efforts required to lift bodies or to support bodies which would otherwise fall to the ground; we speak of these measurable efforts as forces which we exert, and we compare them with the ‘weights’ that we associate with bodies, by which term we mean ‘the forces with which the Earth attracts them’. Thus the phrase ‘a force of x pounds weight’ means a force that would support a body weighing x pounds.

When holding the string of a kite flying in a gusty wind we are conscious of a pull or ‘tension’ on the string at the point where it leaves the hand that holds it, and we realize that this ‘force’ is something measurable and of varying measure, and that it acts now in this direction and now in that as the string moves hither and thither with the kite.