### Simple Machines

Simple machines make work easier, but they do not decrease the amount of work you have to do. Why can’t simple machines change the amount of work that you do? Recall that in closed systems the total amount of energy is conserved. A machine cannot increase the amount of energy you put into it. So, why is a simple machine useful? Although it cannot change the amount of work you do, a simple machine can change the amount of force you must apply to an object, and the distance over which you apply the force. In most cases, a simple machine is used to reduce the amount of force you must exert to do work. The down side is that you must exert the force over a greater distance, because the product of force and distance, **f***d*, (which equals work) does not change.

Let’s examine how this works in practice. In Figure 9.8(a), the worker uses a type of lever to exert a small force over a large distance, while the pry bar pulls up on the nail with a large force over a small distance. Figure 9.8(b) shows the how a lever works mathematically. The effort force, applied at **F**_{e}, lifts the load (the resistance force) which is pushing down at **F**_{r}. The triangular pivot is called the fulcrum; the part of the lever between the fulcrum and **F**_{e} is the effort arm, *L*_{e}; and the part to the left is the resistance arm, *L*_{r}. The mechanical advantage is a number that tells us how many times a simple machine multiplies the effort force. The ideal mechanical advantage, *IMA*, is the mechanical advantage of a perfect machine with no loss of useful work caused by friction between moving parts. The equation for *IMA* is shown in Figure 9.8(b).

In general, the *IMA* = the resistance force, **F**_{r}, divided by the effort force, **F**_{e}.* IMA* also equals the distance over which the effort is applied, *d*_{e}, divided by the distance the load travels, *d*_{r}.

$$IMA=\frac{{F}_{r}}{{F}_{e}}=\frac{{d}_{e}}{{d}_{r}}$$

Getting back to conservation of energy, for any simple machine, the work put into the machine, *W*_{i}, equals the work the machine puts out, *W*_{o}. Combining this with the information in the paragraphs above, we can write

$$\begin{array}{l}{W}_{i}={W}_{o}\\ {F}_{e}{d}_{e}={F}_{r}{d}_{r}\\ \text{If}{F}_{e}{F}_{r}\text{,then}{d}_{e}{d}_{r}\text{.}\end{array}$$

The equations show how a simple machine can output the same amount of work while reducing the amount of effort force by increasing the distance over which the effort force is applied.

### Watch Physics

#### Introduction to Mechanical Advantage

This video shows how to calculate the *IMA* of a lever by three different methods: (1) from effort force and resistance force; (2) from the lengths of the lever arms, and; (3) from the distance over which the force is applied and the distance the load moves.

Grasp Check

Two children of different weights are riding a seesaw. How do they position themselves with respect to the pivot point (the fulcrum) so that they are balanced?

- The heavier child sits closer to the fulcrum.
- The heavier child sits farther from the fulcrum.
- Both children sit at equal distance from the fulcrum.
- Since both have different weights, they will never be in balance.

Some levers exert a large force to a short effort arm. This results in a smaller force acting over a greater distance at the end of the resistance arm. Examples of this type of lever are baseball bats, hammers, and golf clubs. In another type of lever, the fulcrum is at the end of the lever and the load is in the middle, as in the design of a wheelbarrow.

The simple machine shown in Figure 9.9 is called a wheel and axle**.** It is actually a form of lever. The difference is that the effort arm can rotate in a complete circle around the fulcrum, which is the center of the axle. Force applied to the outside of the wheel causes a greater force to be applied to the rope that is wrapped around the axle. As shown in the figure, the ideal mechanical advantage is calculated by dividing the radius of the wheel by the radius of the axle. Any crank-operated device is an example of a wheel and axle.

An inclined plane and a wedge are two forms of the same simple machine. A wedge is simply two inclined planes back to back. Figure 9.10 shows the simple formulas for calculating the *IMA*s of these machines. All sloping, paved surfaces for walking or driving are inclined planes. Knives and axe heads are examples of wedges.

The screw shown in Figure 9.11 is actually a lever attached to a circular inclined plane. Wood screws (of course) are also examples of screws. The lever part of these screws is a screw driver. In the formula for *IMA*, the distance between screw threads is called *pitch* and has the symbol *P*.

Figure 9.12 shows three different pulley systems. Of all simple machines, mechanical advantage is easiest to calculate for pulleys. Simply count the number of ropes supporting the load. That is the *IMA*. Once again we have to exert force over a longer distance to multiply force. To raise a load 1 meter with a pulley system you have to pull *N* meters of rope. Pulley systems are often used to raise flags and window blinds and are part of the mechanism of construction cranes.

### Watch Physics

#### Mechanical Advantage of Inclined Planes and Pulleys

The first part of this video shows how to calculate the *IMA* of pulley systems. The last part shows how to calculate the *IMA* of an inclined plane.

Grasp Check

How could you use a pulley system to lift a light load to great height?

- Reduce the radius of the pulley.
- Increase the number of pulleys.
- Decrease the number of ropes supporting the load.
- Increase the number of ropes supporting the load.

A complex machine is a combination of two or more simple machines. The wire cutters in Figure 9.13 combine two levers and two wedges. Bicycles include wheel and axles, levers, screws, and pulleys. Cars and other vehicles are combinations of many machines.