=83. The Aeroplane.=--The aeroplane consists of one or two frames _ABCD_ (see Fig. 64), over which is stretched cloth or thin sheet metal. It is driven through the air by a propeller turned by a powerful gasoline motor. This has the effect of creating a strong breeze coming toward the front of the aeroplane. As in the case of the sailboat a pressure is created at right angles to the plane along _GF_ and this may be resolved into two components as _GC_ and _GE_, _GC_ acting to lift the aeroplane vertically and _GE_ opposing the action of the propeller. Fig. 65 represents the Curtis Flying Boat passing over the Detroit river. [Illustration: FIG. 65.--The Curtis hydroplane.] Exercises. 1. If a wagon weighing 4000 lbs. is upon a hill which rises 1 ft. in 6, what force parallel to the hill will just support the load? (Find the effective component of the weight down the hill.) 2. If a barrel is being rolled up a 16-ft. ladder into a wagon box 3 ft. from the ground, what force will hold the barrel in place on the ladder, if the barrel weighs 240 lbs. Show by diagram. 3. Show graphically the components into which a man's push upon the handle of a lawn mower is resolved. 4. Does a man shooting a flying duck aim at the bird? Explain. 5. What are the three forces that act on a kite when it is "standing" in the air? 6. What relation does the resultant of any two of the forces in problem five have to the third? 7. Into what two forces is the weight of a wagon descending a hill resolved? Explain by use of a diagram. 8. A wind strikes the sail of a boat at an angle of 60 degrees to the perpendicular with a pressure of 3 lbs. per square foot. What is the effective pressure, perpendicular to the sail? What would be the effective pressure when it strikes at 30 degrees? 9. How is the vertical component of the force acting on an aeroplane affected when the front edge of the plane is elevated? Show by diagram. (4) MOMENT OF FORCE AND PARALLEL FORCES =84. Moment of Force.=--In the study of motion we found that the quantity of motion is called _momentum_ and is measured by the product of the _mass times the velocity_. In the study of _parallel forces_, especially such as tend to produce _rotation_, we consider a similar quantity. It is called a _moment of force_, which is the term applied to the _effectiveness_ of a force in producing change of rotation. It also measured by the product of two quantities; _One, the magnitude of the force itself_, and the other, _the perpendicular distance from the axis about which the rotation takes place to the line representing the direction of the force_. [Illustration: FIG. 66.--The moments about _S_ are equal.]