Physics

by Willis E. Tower; Thomas D. Cope; Charles H. Smith; Charles M. Turton

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.

Fig. 66.—The moments about S are equal.

To illustrate: Take a rod, as a meter stick, drill a hole at S and place through it a screw fastened at the top of the blackboard. Attach by cords two spring balances and draw to the right and left, A and B as in Fig. 66. Draw out the balance B about half way, hold it steadily, or fasten the cord at the side of the blackboard, and read both balances. Note also the distance AS and BS. Since the rod is at rest, the tendency to rotate to the right and left must be equal. That is, the moments of the forces at A and B about S are equal. Since these are computed by the product of the force times the force arm, multiply B by BS and A by AS and see if the computed moments are equal. Hence a force that tends to turn or rotate a body to the right can be balanced by another of equal moment that acts toward the left.