Force is defined as any push or pull that causes an object to change its velocity. When tools or mechanisms are used to perform a task, the forces involved are categorized based on their role within the system. Understanding these categories, particularly input force, is necessary to analyze how devices modify applied effort to achieve a desired result.
Defining Input Force and Output Force
Input force is the effort applied to a machine, typically by the user, to initiate its operation. This force is often called the effort force, and it serves as the energy source that drives the system. Examples include pushing a lever, pulling a rope, or turning a crank.
The input force is immediately contrasted with the output force, which is the force exerted by the machine onto the object, or load, being acted upon. Output force is the result the user is trying to achieve, such as lifting a weight or cutting a piece of wood. In mechanical analysis, the force that the machine works against—the weight of the object or the resistance of the material—is also known as the load force or resistance force.
The core relationship is defined by what the user puts into the machine (input force) versus the result achieved (output force). For example, when using a shovel, the input force is the muscular force applied to the handle. The output force is the force the shovel head exerts on the dirt or sand to move it.
This distinction is important because, while the input force is the effort supplied, the output force is what determines whether the task is successful. A machine’s purpose is often to modify the magnitude or direction of the input force to create a more effective output force. The ratio between these two forces is what defines the efficiency of the machine’s design for a specific task.
The Role of Input Force in Simple Machines
Input force is applied to simple machines to overcome resistance that would be too difficult to manage alone. Simple machines are mechanical devices, such as levers, pulleys, and inclined planes, that change the direction or magnitude of a force. The input force activates the machine, allowing it to apply a force over a distance to complete the task.
The primary reason for applying an input force to a simple machine is to gain mechanical advantage. Mechanical advantage is a measure of how much a machine multiplies the input force. For example, a machine with a mechanical advantage of two means that every unit of input force applied produces two units of output force.
This force multiplication is achieved by applying the input force over a greater distance than the distance the load moves. Because the total work done (force multiplied by distance) must remain constant, a reduction in the required input force is compensated for by an increase in the distance over which that force must be applied. Therefore, a smaller input force can move a much larger load, provided the user moves their end of the machine a proportionally longer distance.
Everyday Examples of Input Force in Action
The concept of input force is demonstrated every time a common household tool is used to make a task easier. Consider using a bottle opener, which functions as a Class 2 lever. The user applies a downward input force to the handle of the opener.
This relatively small downward force is then amplified by the lever’s design, resulting in a much larger upward output force applied directly to the bottle cap. The machine allows the user to overcome the resistance force holding the cap in place with minimal muscular effort.
Another common example is a wheelbarrow, which acts as a lever with the fulcrum at the wheel’s axle. When a person lifts the handles of a loaded wheelbarrow, the upward push they apply is the input force. Since this input force is applied farther from the load than the load is from the axle, the machine generates an output force greater than the effort, making it possible to lift and carry heavy materials.
When using a ramp to move an object, the input force is the force used to push or pull the object parallel to the ramp’s surface. The ramp, an inclined plane, reduces the required input force compared to lifting the object straight up, which is the load force. The trade-off is that the input force must be applied over the entire length of the ramp, which is a much longer distance than the vertical height the object is raised.