An external force is any push or pull acting on an object or system that originates from something outside that system. Gravity pulling you toward the ground, wind pressing against a building, and the road pushing back against a car’s tires are all external forces. The concept sounds simple, but the line between “external” and “internal” shifts depending on how you define the system you’re looking at, and that flexibility is what makes the idea so useful across physics, engineering, biology, and even space exploration.
What Makes a Force “External”
Every force analysis starts with a choice: what counts as “the system”? Once you draw that boundary, any force crossing it from the outside is external, and any force between parts inside that boundary is internal. This isn’t a property of the force itself. The same force can be external or internal depending on how you frame the problem.
Consider two people playing tug of war. If you define one person as your system, the pull from the rope is an external force acting on them. But if you define both people plus the rope as a single system, that same pull becomes an internal interaction. Nothing about the rope or the pull changed. What changed is the boundary you drew around the problem. This boundary-dependence is the single most important thing to understand about external forces, because it explains why the same situation can be analyzed in different ways depending on what question you’re trying to answer.
In practical terms, external forces are the ones that can change a system’s overall motion. Internal forces cancel each other out when you look at the system as a whole. If you’re standing on a skateboard and push against a wall, the wall exerts an external force on the you-plus-skateboard system, and you roll backward. But if you just shift your weight on the board, those are internal forces within the system, and the skateboard’s center of mass stays put.
Common Everyday Examples
Most forces you encounter daily are external forces acting on your body or on objects around you. Recognizing them is easier once you see the pattern: something outside the object is doing the pushing or pulling.
- Gravity: The Earth pulls everything with mass toward its center. When you drop a ball, gravity is the external force accelerating it downward. Even when an object sits motionless on a table, gravity is still acting on it; the table just pushes back with an equal and opposite force.
- Normal force: When you place a book on a desk, the desk exerts an upward push on the book. This “normal force” is perpendicular to the surface and prevents the book from falling through. It’s the desk’s external contribution to the book’s equilibrium.
- Friction: The resistance you feel when you slide a box across the floor comes from microscopic interactions between the surfaces. Friction is an external force that opposes the motion of the object you’re analyzing.
- Applied force: Any time you push a shopping cart, kick a soccer ball, or pull open a door, you’re applying an external force to that object. Your muscles generate the push, but from the object’s perspective, the force arrives from outside itself.
- Air resistance: A skydiver falling through the atmosphere encounters drag from air molecules colliding with their body. This external force increases with speed and eventually balances gravity, which is why skydivers reach a terminal velocity instead of accelerating forever.
- Tension: When you pull a dog on a leash, the leash transmits tension to the dog’s collar. From the dog’s perspective as a system, that tension is an external force.
These examples all follow the same logic. You pick the object, you identify what’s touching it or pulling on it from outside, and those are the external forces. The tricky cases arise when forces are less visible, like air pressure or magnetic fields, but the principle holds.
External Forces on Buildings and Bridges
Structural engineers spend their careers thinking about external forces, because every building, bridge, and dam must be designed to withstand the forces acting on it from outside. The stakes are obvious: get the analysis wrong, and the structure fails.
Wind and earthquakes are the two dominant external forces that tall buildings must resist, and which one matters more depends on the building’s height and location. A literature review of high-rise concrete structures found that for buildings up to roughly 10 to 15 stories, seismic forces usually dominate in earthquake-prone regions, while wind is rarely the critical factor. Above about 20 to 25 floors, especially in areas with strong prevailing winds, the relationship flips. Wind can become the primary external force, causing larger overall displacements than earthquakes would.1ResearchGate / Inżynieria Mineralna. Evaluation of Wind and Seismic Load Modelling in High-Rise Concrete Structures: A Literature Review
That crossover point matters for design. A squat apartment building in California needs robust earthquake resistance but may not require much wind bracing. A 40-story tower on the Gulf Coast faces the opposite priority. Engineers don’t just worry about average winds, either. Gusting, vortex shedding (where wind creates alternating low-pressure zones on each side of a building, causing it to sway rhythmically), and storm surges all count as external forces that the structure must absorb or redirect. Snow loads on roofs, water pressure against a dam face, and the weight of traffic crossing a bridge are additional external forces that enter the calculations.
The key insight is that structural engineers treat the entire building as the system, and everything the environment throws at it, from gravity to wind to ground shaking, is external. Internal forces (the tension in steel reinforcing bars, the compression within concrete columns) are the structure’s response to those external loads. The building survives when its internal forces can match or exceed every external force acting on it.
External Forces in Sports and the Human Body
Biomechanics applies the same framework to the human body. Your muscles generate internal forces within your musculoskeletal system. The ground, equipment, opponents, and gravity supply the external ones. Understanding that distinction helps explain injuries, improve performance, and guide rehabilitation.
When you run, every footstrike generates a ground reaction force, the external push the ground exerts back on your foot. Researchers have spent decades studying these forces as potential predictors of running injuries, though the relationship turns out to be less straightforward than once assumed. A 2024 review of the evidence found only a marginal link between ground reaction forces, tibial acceleration, and actual tibial bone stress, challenging the long-held assumption that higher impact forces automatically mean higher injury risk.2Europe PMC. Rethinking running biomechanics: a critical review of ground reaction forces, tibial bone loading, and the role of wearable sensors The external force is real and measurable, but the body’s internal response to it, how bones and tissues distribute and absorb the load, is what ultimately determines whether an injury occurs.
External forces also play a direct role in ligament injuries. A study using cadaveric knee joints examined how external loads interact with muscle forces at various angles of knee bending. When varus and internal rotation forces (the kind you’d experience during a cutting maneuver on a soccer field or an awkward landing) were applied together from outside the joint, anterior cruciate ligament strain increased significantly across the full range of knee flexion.3PubMed. The influence of muscle forces and external loads on cruciate ligament strain That finding helps explain why ACL tears so often happen during non-contact movements: it isn’t always a collision that does the damage. The external force from the ground, combined with the body’s own momentum and a vulnerable knee angle, can be enough.
Sports equipment is partly designed to manage external forces. Running shoes cushion ground reaction forces. Helmets spread impact forces over a larger area and a longer time. Knee braces resist those varus and rotational loads from the outside. In each case, the engineering goal is to reduce or redistribute the external force before it reaches vulnerable tissue.
External Forces at the Scale of a Single Cell
The concept of external forces doesn’t stop at the scale you can see. Biophysicists study how forces from outside a living cell affect its behavior, shape, and internal chemistry. In this context, the cell membrane is the system boundary, and anything pushing or pulling from beyond it is an external force.
One of the most precise tools for applying external forces to cells is optical tweezers, which use focused laser beams to trap and move microscopic objects. Since their invention more than three decades ago, optical tweezers have opened new avenues for studying the mechanical properties of biological molecules and cells.4PubMed Central. Probing force in living cells with optical tweezers: from single-molecule mechanics to cell mechanotransduction Researchers can grab a tiny bead attached to a cell surface and pull with a known, calibrated force, then watch how the cell responds. Does it stiffen? Stretch? Change its gene expression?
This work has revealed that cells are not passive blobs of fluid. They actively sense and respond to external mechanical forces, a process called mechanotransduction. Your bone cells respond to the external compression of walking by remodeling bone tissue. Your blood vessel walls detect the external shear force of flowing blood and adjust their stiffness accordingly. Tumor cells may respond to the mechanical stiffness of surrounding tissue, which functions as an external force cue, by becoming more aggressive. At this scale, the line between a “force” and a “signal” blurs. An external mechanical push is also information that the cell processes and acts on.
External Forces in Space
Space might seem like the last place you’d worry about external forces, but spacecraft are constantly buffeted by them. The most persistent is solar radiation pressure, the gentle but unrelenting push of sunlight itself. Photons carry momentum, and when they strike or reflect off a spacecraft’s surface, they transfer a tiny force. Over days and weeks, this external force can nudge a satellite off course if it isn’t accounted for.
Engineers have explored creative ways to manage these perturbations. One approach involves using the Lorentz force, the force a magnetic field exerts on a moving charged object. Research has shown that a charged spacecraft moving through Earth’s magnetic field can generate a Lorentz force of a similar magnitude to solar radiation pressure. By carefully controlling the spacecraft’s electrical charge, engineers can use one external force to balance another, keeping the satellite in its intended orbit without burning fuel.5PubMed Central. Balancing the effects of solar radiation pressure on the orbital elements of a spacecraft using Lorentz force
Gravitational forces from the Sun, Moon, and other planets also count as external forces on a spacecraft. Mission planners use these to their advantage: gravity assists, where a probe swings past a planet and steals a bit of its orbital energy, are a deliberate exploitation of an external gravitational force to change speed and direction without using propellant. The Voyager probes used this technique to visit the outer planets on trajectories that would have been impossible with onboard fuel alone.
Even in deep space, far from any planet, the solar wind (a stream of charged particles from the Sun) exerts a small external force on anything in its path. Proposed solar sails would harness this force intentionally, using enormous reflective surfaces to accelerate without any engine at all. The force per square meter is tiny, but in the frictionless vacuum of space, even a tiny continuous external force produces steady acceleration that compounds over time.
Why the System Boundary Matters More Than the Force
A common misconception is that certain forces are inherently external. Gravity seems like it should always be external, friction always external, muscle force always internal. But none of that holds universally. Gravity is external to your body, but internal to the Earth-plus-you system. Friction between your shoe and the ground is external to your foot, but internal to the shoe-plus-ground system. A muscle force is internal to your body, but external to the individual bone the muscle pulls on.
This flexibility is the whole point. Physicists and engineers choose system boundaries strategically to simplify the problem they’re trying to solve. If you want to predict how a car accelerates down a highway, you define the car as the system and treat the road’s friction, air drag, and gravity as external forces. If you want to understand why the engine overheats, you might zoom in and define just the engine block as the system, in which case the combustion gases themselves become an external force acting on the piston walls.
The same logic extends to every scale. An economist might treat a national economy as a system and call foreign trade an “external force” on GDP. A psychologist might treat an individual’s mental state as the system and call social pressure an external force. The physics definition is precise, involving measurable pushes and pulls, but the conceptual pattern shows up everywhere people analyze cause and effect.
How External Forces Relate to Newton’s Laws
External forces are the forces that actually show up in Newton’s second law when you’re analyzing a system’s motion. The law says that a system’s acceleration equals the net external force on it divided by its mass. Internal forces, no matter how large, don’t appear in that equation because they come in equal and opposite pairs that cancel out when you sum across the whole system.
This is why a car can’t accelerate by having the engine push against its own chassis. The engine pushes the chassis forward, and the chassis pushes the engine backward, and the net is zero. The car accelerates because the tires push backward on the road (internal, from the car’s perspective, if you include the tires in the system), and the road pushes forward on the tires (external). That external push from the road is what changes the car’s velocity.
Conservation of momentum works the same way. A system’s total momentum changes only when a net external force acts on it. Two ice skaters pushing off each other exchange momentum internally, and the total for the pair stays constant. But if one skater also pushes against the rink wall, that wall provides an external force that changes the pair’s combined momentum. These laws aren’t just classroom exercises. They’re the reason aerospace engineers track every external perturbation on a satellite, and the reason a structural engineer cares about every gust of wind and tremor from below.
Forces That Blur the Line
Some situations make the external-versus-internal distinction genuinely tricky. Consider a person standing inside an elevator. If your system is the person alone, the elevator floor’s push is an external normal force. If your system is the person plus the elevator car, that push becomes internal, and the cable tension pulling the car upward is now the relevant external force. But what if the cable is part of the system too? Then you need to extend all the way to the motor, and eventually to the building’s foundation pushing against the Earth. Every time you widen the boundary, forces that were external get absorbed inside, and you need new external forces at the new boundary to explain the motion.
Fluid forces present another subtlety. When water flows through a pipe, is the pressure an internal or external force? If the system is the water inside a specific section of pipe, the pressure from water upstream and downstream is external (it crosses the boundary of your chosen section). If the system is all the water in the pipe, only the pump and the resistance at the outlet are external. Hydraulic engineers switch between these framings constantly, depending on whether they’re analyzing local turbulence or overall flow rate.
Electromagnetic forces add yet another layer. A charged particle in a magnetic field experiences an external force (the Lorentz force mentioned earlier in the spacecraft context). But if you define your system as the particle plus the magnetic field source, the interaction becomes internal. In plasma physics, where billions of charged particles create their own collective fields while responding to external ones, deciding where “external” ends and “internal” begins is one of the central challenges of the field.
These aren’t failures of the concept. They’re evidence that the external-versus-internal distinction is a tool, not a fixed property of nature. The tool works when you choose boundaries that isolate the question you need answered, and it creates confusion only when the boundary is ambiguous or poorly chosen.