Equilibrium

With what can we associate the word "equilibrium"? We ought to know that the term applies equally well to a motor­car moving along a straight road at a constant speed. From a scientific point of view an object can be in equilibrium not only when it is standing motionless, but also when it is moving in a straight line at constant speed. On the other hand would not be in equilibrium if it were increasing speed slowing down or provided it were going around a corner.

Evidently, when there are no forces pulling or pushing5 body, that body must always be in equilibrium. Howeve objects with forces acting on them may also be in equilibriuf But, then, the forces must balance; that is, their vector svi is to equal zero. Thus, in the case of the speeding automobi there are forces acting, but they balance each other in pai The pull of gravity downward is counteracted by the push the road upward. The driving force supplied by the motorf balanced by air resistance and other frictional forces. Ho] wever, as the driving force supplied by the motor is greatej than the frictional resistance, the car is accelerated.

But sometimes an object may not be in equilibrium evel though the vector sum of all the forces is zero. It may have a tendency to rotate unless all the forces are applied ate single point, or unless the tendency to rotate in one direction balances the tendency to rotate in the opposite.

Thus the second condition for equilibrium is that the tendency to rotate should be zero. There are always two requirements for equilibrium: first, the vector sum of all the forces acting on a body should be zero; and, second, there must be no tendency to rotate, that is, the tendency to rotate the body in one direction must be balanced by the tendency to rotate it in the opposite direction.

We have to point out the importance of equilibrium in anf engineering design and construction. For example, if a house is to stand, each of its parts must evidently be in equilibrium-