An internal force is any force that one part of a system exerts on another part of the same system. If you define a box around a group of objects or particles, every push or pull happening entirely inside that box counts as an internal force. These forces always come in equal and opposite pairs, so they cancel each other out when you look at the system as a whole. That cancellation is what makes internal forces fundamentally different from external forces, which come from outside the system and can change its overall motion.
How Internal Forces Differ From External Forces
The distinction between internal and external forces depends entirely on where you draw your system’s boundary. A force is internal if both the object doing the pushing and the object being pushed are inside your defined system. A force is external if it comes from something outside the system acting on something inside it.
Here’s a classic example: your car’s engine spins the wheels, but the engine alone doesn’t propel the car forward. For the system of “car plus engine plus wheels,” the engine turning the axle is an internal force. What actually accelerates the car is static friction between the tires and the road, which is an external force because the road is outside the system. Similarly, you can’t lift yourself off the ground by standing in a basket and pulling up on the handles. For the system of you plus the basket, your pulling force is internal. It can’t change the motion of the whole system.
This is the key insight: internal forces never change the acceleration of a system as a whole. Only external forces do that. When you add up every force acting on every particle in a system, all the internal forces sum to zero, leaving only the external forces to determine how the system moves. This follows directly from Newton’s Third Law: whenever particle A pushes on particle B, particle B pushes back on A with equal force in the opposite direction. Those two forces cancel perfectly.
Why Internal Forces Still Matter
The fact that internal forces cancel out across an entire system doesn’t mean they’re unimportant. They determine what happens inside the system. Internal forces can rearrange parts, deform materials, store energy, and cause individual components to speed up or slow down even when the system’s overall momentum stays the same.
When two billiard balls collide, the forces between them are internal to the two-ball system. The system’s total momentum doesn’t change, but each individual ball’s speed and direction do. When a bomb explodes, the internal forces send fragments flying in every direction, yet the center of mass of all those fragments continues along the same path it was following before the explosion.
Internal forces also play a central role in energy. The work done by all conservative internal forces equals the negative of the change in the system’s potential energy. A compressed spring stores potential energy through the internal forces between its coils. When it releases, those internal forces convert that stored energy into kinetic energy of the objects attached to it.
Types of Internal Forces in Structures
In engineering, internal forces take on very specific meanings. When external loads act on a structure like a bridge, beam, or building column, the material develops internal forces that resist those loads. Engineers categorize these into several types.
- Tension: Two opposing pulling forces that try to stretch a member apart. A rope supporting a hanging weight is in tension, as is a chain towing a car.
- Compression: Two opposing pushing forces that try to squeeze a member together. Standing on a soda can subjects it to compression. Building columns carry compressive loads from the floors above.
- Shear: Forces that act parallel to a surface, sliding one layer of material past another. Scissors cutting paper and pruning shears cutting a branch both apply shear loads.
- Bending: A combination of tension and compression caused by a force acting at a distance from a support point, creating a “turning force” called a moment. A diving board bends under a diver’s weight.
- Torsion: A twisting force around the long axis of a member. Turning a wrench on a bolt subjects the bolt to torsion.
In three dimensions, a structural member like a beam or column can experience up to six internal force components at any cross-section: one normal (axial) force along its length, two shear forces perpendicular to its length, two bending moments, and one torsional moment. Engineers analyze these to ensure that the stresses inside the material never exceed what it can safely handle.
Internal Forces in the Human Body
Your body is a mechanical system, and internal forces are at work every time you move. Muscles generate pulling forces that act on bones through tendons. Ligaments transmit forces from bone to bone, stabilizing joints and guiding their motion. All of these are internal forces within the system of your body.
When you lift a heavy box, your biceps pull on your forearm bones, your spinal muscles resist the bending load on your back, and your leg muscles push against your femur and tibia to keep you upright. None of these forces can lift your entire body off the ground on their own. What keeps you standing is the external force: the ground pushing up on your feet. The internal forces simply distribute loads and control the positions of your limbs relative to each other.
The System Boundary Changes Everything
One of the trickiest parts of understanding internal forces is recognizing that the same force can be internal or external depending on how you define your system. If your system is just one wheel of a car, the force from the axle on that wheel is external. If your system is the entire car, that same axle force is internal.
This flexibility is actually a powerful tool. Physicists and engineers choose system boundaries strategically to simplify problems. Want to find how fast a car accelerates? Define the whole car as your system and focus on external forces like friction, air resistance, and gravity. Want to know if an axle will break? Zoom in, make the axle your system, and the forces from the engine and wheels become external forces you can analyze directly.
The underlying physics never changes. What changes is your perspective, and choosing the right perspective is often the difference between a problem that takes five minutes and one that takes an hour.