1. Learn the notions and their definitions.

Pressure The force per unit area.

Work Moving some object by force through a distance

Energy Quantity of work done in a minute af time

Power The capacity of a system or body for doing work

Friction The resistance to the horizontal force applied to the body at rest on the horizontal surface;

Coefficient of Friction The constant ratio of the friction to the

force pressing the surfaces together.

Static Friction The resistance to motion between two bodies in contact with each other but at rest.

Sliding Friction The resistance to continued motion when one body has started to move.

Rolling Friction The resistance to motion when one or more bodies

Elasticity The property of bodies of returning, after unloading, to their initial form

Joule An international metric measure of energy; one joule is ap­proximately three-quarters of a foot-pound.

Watt An international metric measure of power equal to one joule per second; 746 watts are equal to one horsepower.

Kilowatt A thousand watts.

Foot-pound Work is usually measured in terms of foot-pounds in the English-speaking countries. One foot-pound is a force of one pound through a distance of one foot in the direction of the force.

2. Read the text to find illustrations of the notions in the title.

Text 1

Statics. Force. Equilibrum. Pressure

An action of one body upon another which changes or tends to change the rate or direction or speed of bodies is called force. A force has magnitude, direction, and place of application; when the extent of the place of application is negligible and the force is regarded as applied or concentrated at a point, a line through the point parallel to the direction of the force is the line of action. Any number of forces considered col­lectively is a system of forces; a system is concurrent or non-concurrent, according

as the lines of action of the forces do or do not intersect in a point, and it is coplanar, or noncoplanar, according as they do or do not lie in a plane. The resultant of a system of forces is equivalent to that system, a resultant includes a single and never more than two forces. The process of determining the resultant is called composition.

A body is formally regarded as rigid if the distance be­tween any set of two points in it is always constant. In reality no body is perfectly rigid. When equal and opposite forces are applied to a body, it is always deformed slightly compressed or stretched. The body's own tendency to restore the deformation has the effect of applying counterforces to whatever is apply­ing the forces, thus obeying Newton's third law. Calling a body rigid means that the changes in the dimensions of the body are small enough to be neglected.

If magnitudes, lines of action, and senses of two concurrent forces

acting on a rigid body are represented by OA and OB (see Fig.1), then the magnitude, line of action, and sense of the resultant is represented by the diagonal OC of the parallelogram OBCA. The points of application of the forces may be anywhere on the body in the lines OA, OB, and OC, or their extensions. If the magnitudes and directions of the concurrent forces are represented by AB and BC (see Fig. 2), then the magnitude and direction of the resultant is represented by the side AC of the triangle ABC.

O

Fig. 1 Fig. 2

Equilibrum. Statics is the study of bodies and structures that are in equilibrium. For a body to be in equilibrium, there must be no net force acting on it. In addition, there must be no net torque acting on it. The torque on a body due to a given force depends on the reference point chosen, since the torque r by defini­tion equals r•f, where r is a vector from some chosen reference point to the

point of application of the force.

Pressure is defined as the force per unit area. Pressure acts equally in all directions. The pressure on the body immersed in liquid is due to the weight of a liquid and may be calculated easily for any depth. Pressure 10∙1 or 10 tons is the force with which the one-square-metre column estabishes on the bottom of the tank and is, therefore, the pressure in tons per square metre.