What is a common characteristic of all simple machines?

A common characteristic of all simple machines is that they help do work more easily. Simple machines are designed to either increase the force applied, change the direction of the force, or allow for easier movement of an object. This fundamental principle is what makes simple machines valuable tools for accomplishing tasks that would otherwise require more effort.

For example, a lever allows a person to lift a heavy object with less force by using a fulcrum to change the direction of the applied force. A pulley simplifies lifting by allowing a person to lift a load vertically by using a rope that changes direction. Similarly, inclined planes allow objects to be raised with less force over a longer distance.

The other characteristics mentioned in the choices do not apply universally to simple machines. While friction can affect the efficiency of a simple machine, it is not a defining feature that applies to all. Not all simple machines necessarily include moving parts; for instance, an inclined plane is a simple machine that does not involve moving parts at all. Lastly, while many simple machines require human effort to operate, they can also be used in systems where other forms of energy, such as mechanical or gravitational, are applied. Thus, the key defining characteristic of simple machines is their ability to make work easier.