Let’s examine how this works in practice. 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. In most cases, a simple machine is used to reduce the amount of force you must exert to do work.

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. A machine cannot increase the amount of energy you put into it. 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. Simple machines make work easier, but they do not decrease the amount of work you have to do. This is the underlying principle of all simple machines. Because the amount of work is not changed, the term f d does not change, but force can decrease if distance increases. Explain that the product of force and distance is critical to understanding simple machines. Explain that simple machines are often hand-held, and that they reduce force, not work. Be sure students do not equate machines and motors by asking for (and, if necessary, providing) examples of machines that are not motorized. Dispel any misconceptions that machines reduce the amount of work.

Ask the students what they know about machines and work. In this section you will apply what you have learned about work to find the mechanical advantage and efficiency of simple machines.

Mechanical advantage word problems