Transportation on Land 255 



crum; the distance from the fulcrum to the apparent center 

 of the weight, which is one arm, and the distance from the ful- 

 crum to the point of application of force, which is the other arm. 

 The arms are distinguished as the weight or resistance arm and 

 the force arm. In the case of the primitive vehicle, the ful- 

 crum, the weight arm, and the force arm are easily identified. 

 Imagine the crude conveyance fitted with a wheel, the axle of 

 which is the fulcrum, and you have a sort of wheelbarrow. 

 What are the weight arm and the force arm of a wheelbarrow ? 

 Imagine now that two such simple levers are joined by their 

 fulcrums on a common axis, and you at once recognize that 

 the resulting machine resembles the nutcracker, one kind of 

 lemon squeezer, and many other familiar tools. All of these 

 are applications of the principle of the lever. 



The fulcrum, the weight, and the force do not always occupy 

 the relative positions shown in the drag and the wheelbarrow. 

 The weight may be located at one end of the lever, and the 

 fulcrum at any point between the weight and the force. Thus 

 the crowbar may be used as such a lever or pry in moving 

 heavy bodies. The principle involved in this use of the lever 

 is exactly the same as in its use as a lifting machine (Fig. 75). 



An example will make this clear. A stone weighing 450 

 pounds is to be moved by a pry. If the length of the weight 

 arm is six inches and the length of the force arm 4.5 feet, what 

 amount of force will be required? The relation of force and 

 weight is exactly that stated above. The product of the 

 weight and the weight arm is equal to the product of the force 

 and the force arm ; or 450 X .5 = Force X 4.5, or 50 pounds, 

 the amount of force required. 



If the fulcrum is exactly midway between the centers of 

 force and of weight, the weight and force must be equal. This 

 is illustrated in the trip scales, or the teeter-board with equal 

 weights on the ends. Both of these are equal-arm levers. 

 With unequal weights the arms of the teeter-board are of 

 unequal length, as shown in Fig. 73. 



