8. A ladder 30 ft. long and weighing 80 lbs. leans against the side of a building so that it makes an angle of 30 degrees with the building. Find the direction and magnitude of the component forces on the ground and at the building. 9. A traveling crane 50 ft. long weighing 10 tons moves from one end of a shop to the other, at the same time a load of 4000 lbs. moves from end to end of the crane. Find the pressure of the trucks of the crane on the track when the load is at a distance of 5, 10, 15, and 25 ft. from either end. [Illustration: FIG. 71--A truss.] 10. Resolve a force of 500 lbs. into two components at right angles to each other, one of which shall be four times the other. 11. A truss (see Fig. 71), carries a load of 1000 lbs. at _C_. Find the forces acting along _AC_, _BC_, and _AB_. If _AC_ and _BC_ are each 12 ft. and _AB_ 20 ft., which of these forces are tensions and which are pressures? (5) GRAVITATION AND GRAVITY =87. Gravitation.=--Gravitation is the force of attraction that exists between all bodies of matter at all distances. This attraction exists not only between the heavenly bodies, the stars and planets, etc., but is also found between bodies on the earth. A book attracts all objects in a room and outside of a room as well, since its weight shows that it is attracted by the earth itself. The gravitational attraction between ordinary bodies is so slight that it requires careful experiments to detect it. In fact, it is only when one of the attracting bodies is large, as for example the earth, that the force becomes considerable. Careful studies of the motions of the heavenly bodies, especially of that of the moon in its orbit about the earth, led Sir Isaac Newton to the statement of the _law of gravitation_ which is well expressed in the following statement: =88. Law of Gravitation.=--_Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them._ The law may be separated into two parts, one referring to the masses of the bodies concerned, the other to the effect of the distance between them. The first part is easily understood since we all know that two quarts of milk will weigh just twice as much as one quart. To illustrate the second part of the law, suppose that the moon were removed to _twice_ its present distance from the earth, then the attraction between the earth and the moon would be _one-fourth_ its present attraction. If removed to _three_ times its present distance, the attraction would be _one-ninth_, etc.