Virtually everyone knows what a lever is, although most people might be surprised to learn just how wide a range of simple machines qualify as such.
Loosely speaking, a lever is a tool that is used to "pry" something loose in a way that no other non-motorized apparatus can manage; in everyday language, someone who has managed to gain a unique form of power over a situation is said to possess "leverage."
Learning about levers and how to apply the equations pertaining to their use is one of the more rewarding processes introductory physics offer. It includes a little bit about force and torque, introduces the counter-intuitive but crucial concept of multiplication of forces, and dials you in to core concepts such as work and forms of energy in the bargain.
One of the main advantages of levers is that they can be easily "stacked" in such a way as to create a significant mechanical advantage. Compound lever calculations help illustrate just how powerful yet humble a well-designed "chain" of simple machines can be.
Fundamentals of Newtonian Physics
Isaac Newton (1642–1726), in addition to being credited with co-inventing the mathematical discipline of calculus, expanded on the work of Galileo Galilei to develop formal relationships between energy and motion. Specifically, he proposed, among other things, that:
Objects resist changes to their velocity in a manner proportional to their mass (the law of inertia, Newton's first law);
A quantity called force acts on masses to change velocity, a process called acceleration (F = ma, Newton's second law);
A quantity called momentum, the product of mass and velocity, is very useful in calculations in that it is conserved (i.e., its total amount doesn't change) in closed physical systems. Total energy is also conserved.
Combining a number of the elements of these relationships results in the concept of work, which is force multiplied through a distance:
It is through this lens that the study of levers begins.
Overview of Simple Machines
Levers belong to a class of devices known as simple machines, which also includes gears, pulleys, inclined planes, wedges and screws. (The word "machine" itself comes from a Greek word that means "help make easier.")
All simple machines share one trait: They multiply force at the expense of distance (and the added distance is often cleverly hidden). The law of conservation of energy affirms that no system can "create" work out of nothing, but even if the value of W is constrained, the other two variables in the equation are not.
The variable of interest in a simple machine is its mechanical advantage, which is just the ratio of the output force to the input force:
Often, this quantity is expressed as ideal mechanical advantage, or IMA, which is the mechanical advantage the machine would enjoy if not frictional forces were present.
Lever Basics
A simple lever is a solid rod of some sort that is free to pivot about a fixed point called a fulcrum if forces are applied to the lever. The fulcrum can be located at any distance along the length of the lever. If the lever is experiencing forces in the form of torques, which are forces acting about an axis of rotation, the lever will not move provided the sum of the forces (torques) acting on the rod is zero.
Torque is the product of an applied force plus the distance from the fulcrum. Thus a system consisting of a single lever subject to two forces F_{1} and F_{2} at distances x_{1} and x_{2} from the fulcrum is in equilibrium when F_{1}x_{1} = F_{2}x_{2}.
- The product of F and x is called a moment, which is any force that compels an object to begin rotating in some way.
Among other valid interpretations, this relationship means that a strong force acting over a short distance can be precisely counterbalanced (assuming no energy losses due to friction) by a weaker force acting over a longer distance, and in a proportional manner.
Torque and Moments in Physics
The distance from the fulcrum to the point at which a force is applied to a lever is known as the lever arm, or moment arm. (In these equations, it has been expressed using "x" for visual simplicity; other sources may use a lowercase "l.")
Torques do not have to act at right angles to levers, though for any given applied force, a right (that is, 90°) angle yields the maximum amount of force because, to simply the matter somewhat, sin 90° = 1.
For an object to be in equilibrium, the sums of the forces and the torques acting on that object must both be zero. This means that all clockwise torques must be balanced exactly by counterclockwise torques.
Terminology and Types of Levers
Usually, the idea of applying a force to a lever is to move something by "leveraging" the assured two-way compromise between force and lever arm. The force you are trying to oppose is called the resistance force, and your own input force is known as the effort force. You can thus think of the output force as reaching the value of the resistance force at the instant the object starts to rotate (i.e., when equilibrium conditions are no longer met.
Thanks to the relationships between work, force and distance, MA can this be expressed as
Where d_{e} is the distance the effort arm moves (rotationally speaking) and d_{r} is the distance the resistance lever arm moves.
Levers come in three types.
- First-order: The fulcrum is between the effort and resistance (example: a "see-saw").
- Second-order: The effort and the resistance are on the same side of the fulcrum, but point in opposite directions, with the effort farther from the fulcrum (example: a wheelbarrow).
- Third-order: The effort and the resistance are on the same side of the fulcrum, but point in opposite directions, with the load farther from the fulcrum (example: a classic catapult).
Compound Lever Examples
A compound lever is a series of levers acting in concert, such that the output force of one lever becomes the input force of the next lever, thus allowing ultimately for a tremendous degree of force multiplication.
Piano keys represent one example of the splendid results that can arise from building machines that feature compound levers. An easier example to visualize is a typical set of nail clippers. With these, you apply force to a handle that draws two pieces of metal together thanks to a screw. The handle is joined to the top piece of metal by this screw, creating one fulcrum, and the two pieces are joined by a second fulcrum at the opposite end.
Note that when you apply force to the handle, it moves much farther (if only an inch or so) than the two sharp clipper ends, which only need to move a couple of millimeters to close together and do their job. The force you apply is easily multiplied thanks to d_{r} being so small.
Lever Arm Force Calculation
A force of 50 newtons (N) is applied clockwise at a distance 4 meters (m) from a fulcrum. What force must be applied at a distance 100 m on the other side of the fulcrum to balance this load?
Here, assign variables and set up a simple proportion. F_{1}= 50 N, x_{1} = 4 m and x_{2} = 100 m.
You know that F_{1}x_{1} = F_{2}x_{2}, so
Thus only a tiny force is needed to offset the resistance load, as long as you are willing to stand the length of a football field away to get it done!
References
- Georgia State University Hyperphysics: Simple Machines
- LibreTexts Physics: Simple Machines
- Texas Gateway (TEA): Simple Machines
- Georgia State University Hyperphysics: Torque and Equilibrium
- Massachusetts Institute of Technology: What Is a Moment?
- University of Oregon: Newtonian Physics
- Engineering Toolbox: Levers
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.