Question

Scissors are like a double-lever system. Which of the simple machines inFigure 9.23andFigure 9.24is most analogous to scissors?

Short answer

Figure 9.23 is analogous to scissors.

Step-by-step solution

The Free-body Diagram of 9.23

The force applied by the nail puller is shown by$F_{\circ }$. The reaction forces$F_n$exerted by the nail on the puller are equal and opposite to the applied force $F_{\circ }$. The distance from the lever arms is $l_{\circ }$and $l_i$. We can conclude that this nail puller is a first-class lever.

The free-body diagram of 9.24

In figure 9.24, in the case of the wheelbarrow, the pivot point is the axle. The load is shown in the figure. In this case, the output force is greater than the input force.

Thus, a wheelbarrow will enable the lifting of heavy loads. We can conclude that the wheelbarrow is a second-class lever.

The Forces on Shovel

In the case of the shovel, the input force is between the pivot and the load. In this case, the lever arm is short. The output force is shorter than the input force as the input force is exerted closer to the pivot.

Thus we can conclude that the shovel is a second-class lever.

The Forces on a Scissor

Scissors are defined as the combination of two first-class levers. The fulcrum/joint is between the load and the effort.

Thus, the scissors resemble figure 9.23, the nail puller.