Matter, force, motion, and friction

Matter, force, motion, and friction

The concept of matter.

We know that physics began with the more or less qualitative passive observation of obvious natural phenomena, such as the downward motion of a freely falling body, the tides of the ocean, the lightning, the rainbow, the mysterious behaviour of magnets, etc. Very early there then came to be looked upon as properities of this basics, i .e., the thing called matter. Just what matter is, was never quite clear. Even today we cannot say with confidence what matter is, but we still talk about a great many physical phenomena as properities of matter, and with the greatly increased study of atomic structure we feel that we are some reaching gradually a clearer understanding of the constitution of matter.

The states or phases of matter.

It is customary to divide matter into three groups in accordance with the appearance it presents to the eye and the effect it has on the othes sensess. These groups are known as states or phases. At any given temperature, e.g. , that of the room (about 20°C), certain substances take the solid form, others are in the liquid phase, and still others are gaseous. We are all familiar with the general characteristics of these phases: how solids have a definite volume nad shape which are not readily changed save by the application of stresses, while a liquid takes the shape of the vessel in which it is placed, although its volume is definite and it possesses a free boundary surface, Finally, a gas takes both its volume and its shape from the contaning vessel and has no free surface.

Force and motion.

Acceleration is always produced by force.

Whenever a body is acceleratinh, a force is acting upon it to cause the acceleration. Thus, a force is aplied to a body to set in motion, A gain, a force is aplied to a body already in motion in other to speed it up, slow it down, or change its direction, Any change in velocity implies an acceleration, and this acceleration can be produces only by force.

A force can be described as a push or a pull acting on a body. Although a force must act upon a body when it accelerates, it does not necessarily follow that a body will accelerate whena force acts upon it.  When all of the forces acting upon a body are taken into account and these do not balancem then the u alanced force is the resultant force acting on the body; this resultant always causes acceleration.

Motion of a rigid body.

We have learned that forcess applied to a body cause the body as a whole to accelerate in the directlion of the resultant force. Such motion, in which every particle of the body has the same velocity and the same acceleration, is called translation, But a torque acting upon a body may produce an entirely different kind of motion, in which the particles of the body describe concentric circles about a fixed line as an axis, a motion known as rotation. In generalm a body may undergo translation or rotation, or a combination of these motions, depending on how the forces are applied with respect to a particular point called the center of mass of the body.

Force and friction.

The surface of any solid if magnified sufficiently is found to be rugged and uneven, with minute hills at some places and valleys at others. If two surfaces are pressed together, these irregularities intermesh somewhat. Because of these surface irregularities, opposition is encountered to the sliding of one surface over another. This oposition, called sliding friction, incrrases as the contacting surfaces are pressed more firmly together. Experiments with metal surfaces in sliding contact indicate that increased pressure may result in high local temperatures at the contacting areas, followed by siezure, in which portions of the two surfaces become welded together, with the subsequent tearing away of relatively large particles. A frictionless or so-called smooth surface represents an ideal that is never attained on partice. Notwithstanding its disadvantages, friction has ceratin useful aspects; indeed, a person could not walk on the ground without friction.