In science, a load is any force, weight, or demand placed on a system. The term shows up across nearly every branch of science and engineering, but the core idea stays the same: something is being pushed, pulled, stressed, or asked to do work. What changes is the context. In physics, a load is a force acting on a structure. In electrical engineering, it’s a device consuming power. In environmental science, it’s the amount of pollution entering a body of water. Understanding which type of load someone is referring to depends entirely on the field.
Load as a Force in Physics
At its most basic, a load is the force exerted on a surface or body. If you place a heavy box on a table, the weight of that box is the load acting on the table. The standard unit for measuring this kind of mechanical load is the newton, which is the force needed to accelerate one kilogram of mass at one meter per second squared.
An important distinction: load and stress are not the same thing. A load is the external force applied to an object. Stress is what happens inside the material as a result. When you squeeze a rubber ball, the force from your hand is the load, and the internal resistance within the rubber is the stress. Strain, meanwhile, is a measure of how much the object deforms (stretches, compresses, or bends) under that load.
Dead Loads vs. Live Loads in Engineering
Structural engineers split loads into two major categories. Dead loads are the weight of the permanent, nonmoveable parts of a structure. For a bridge, that includes the towers, cables, and roadway itself. These forces never change as long as the structure exists.
Live loads are everything else: temporary, moveable forces acting on the structure. Traffic driving across a bridge, people walking on a floor, furniture in a building. Environmental forces like wind, rain, snow, and earthquakes also count as live loads because they affect the structure temporarily. Engineers have to design for worst-case combinations of both types to keep a building or bridge standing safely.
Electrical Loads in Circuits
In electrical engineering, a load is any component or device that consumes electric power. The lights in your house, a toaster, a laptop charger: all of these are electrical loads. The term can also refer to the total power consumed by an entire circuit.
More technically, if a circuit has an output, whatever is connected to that output is the load. Think of plugging a speaker into an amplifier. The speaker is the load on the amplifier’s output circuit. Electrical loads can be resistive (like a heater, which converts electricity directly to heat), capacitive (which stores and releases energy in electric fields), or inductive (which stores energy in magnetic fields, like a motor). The type of load affects how the circuit behaves and how efficiently it delivers power.
Loads on the Human Body
In biomechanics and exercise science, a load describes physical stresses acting on the body or on structures within it. Your bones, muscles, tendons, ligaments, and intervertebral discs all experience mechanical loads every time you move, lift something, or even just stand upright.
External loads come from the physical environment: the weight of an object you’re carrying, the impact of your foot hitting the ground while running, or the resistance of a barbell during a squat. These external forces travel through your limbs and body to create internal loads on tissues. Bones are the primary load-bearing tissue. They handle gravitational forces and the pull of muscles during movement. When forces compress or bend bones repeatedly over time, the bones adapt by becoming denser and stronger. But excessive or repetitive loading without adequate recovery can damage tissues, contributing to injuries in ligaments, tendons, muscles, and nerves.
Pollutant Load in Environmental Science
Environmental scientists use “load” to describe the mass of a pollutant transported into a body of water over a specific period of time. If a river receives 50 kilograms of nitrogen runoff per day from surrounding farmland, that’s the pollutant load. The loading rate (or flux) is the instantaneous rate at which the pollutant passes a given point, measured in units like grams per second or tons per day.
This concept is central to water quality regulation. Under the Clean Water Act, states must establish a Total Maximum Daily Load (TMDL) for polluted waterways. A TMDL is the maximum amount of a specific pollutant that a body of water can receive while still meeting water quality standards. The calculation accounts for all pollution sources, both point sources (like factory discharge pipes) and nonpoint sources (like agricultural runoff), and includes a margin of safety to handle uncertainty. States are required to develop TMDLs for every waterway on their list of impaired waters.
System Load in Computing
In computer science, load refers to the amount of computational work a system is performing. On Unix and Linux systems, the load number represents how many processes are currently using or waiting to use the CPU. An idle computer has a load of zero. Each active process adds one to the count.
Linux takes this a step further by also counting processes stuck waiting for disk activity, not just those waiting for the CPU. This means a computer with a slow or overloaded hard drive can show a high load number even if the processor itself isn’t busy. System administrators monitor load averages (typically reported over 1-minute, 5-minute, and 15-minute windows) to gauge whether a server is handling its workload well or needs more resources. Research on load balancing has found that measuring CPU queue length, the number of processes waiting in line, is a more effective way to distribute work across multiple computers than simply measuring how busy each CPU is.
Why the Same Word Means So Many Things
The thread connecting all these definitions is the idea of demand placed on a system. A bridge bears the load of traffic. A circuit bears the load of connected devices. A river bears the load of pollutants. Your skeleton bears the load of gravity and movement. A computer bears the load of running programs. In every case, the load is whatever the system has to handle, and the science of each field is largely about understanding how much load a system can take before something breaks, fails, or degrades.