In AP Physics 1, internal forces are forces that objects within a defined system exert on each other; because they come in Newton's third law pairs, they cancel out and cannot change the motion of the system's center of mass or the system's total momentum.
An internal force is any force exchanged between two parts of the same system. Whether a force counts as internal depends entirely on where you draw the system boundary. The tension in a string connecting two blocks is internal if your system is "both blocks plus the string," but external if your system is just one block.
Here's the payoff. Every internal force comes paired with an equal and opposite partner (Newton's third law), and both members of the pair act inside the system. They cancel when you add up all the forces. That means internal forces can never accelerate the system as a whole or change its total momentum. They can deform the system, redistribute energy inside it, or change how parts move relative to each other, but the center of mass only responds to external forces. This one idea is why a car can't push itself forward by having passengers shove the dashboard, and why momentum is conserved in collisions when you treat both colliding objects as one system.
Internal forces sit at the heart of system selection, a skill the AP Physics 1 course makes you practice constantly in Unit 2 (Force and Translational Dynamics) and Unit 4 (Linear Momentum). When you write Newton's second law, you only include forces external to your chosen system, so deciding what's internal versus external is step one of every dynamics problem. The same logic powers conservation of momentum in Unit 4. Momentum is conserved precisely when the net external force on the system is zero, which means all the interesting forces (like collision forces between two carts) are internal. If you can correctly label a force as internal or external, you've already done the conceptual heavy lifting on most multi-object problems.
Keep studying AP Physics 1 Unit 2
External Forces (Unit 2)
Internal and external are two sides of the same system-boundary coin. The exact same force, like tension in a connecting string, flips from external to internal the moment you redraw your system to include both objects. Only external forces show up in Newton's second law for the system.
Atwood's Machine (Unit 2)
Atwood and modified Atwood setups are the classic playground for this idea. Treat the two blocks and string as one system, and tension becomes internal, so the system's acceleration is just the net external force (gravity on the hanging block) divided by total mass. Treat one block alone, and tension reappears as an external force you must solve for.
Tension (Unit 2)
Tension is the most common force that switches between internal and external on the exam. A string pulls equally on both objects it connects, so when both objects are in your system, the two tension forces cancel and vanish from the analysis.
Conservation of Linear Momentum (Unit 4)
Collisions work because the huge forces the objects exert on each other are internal to the two-object system. They cancel as a third-law pair, so total momentum before equals total momentum after. Internal forces are literally the reason conservation of momentum exists.
You won't usually see a question that asks "define internal force." Instead, the exam tests whether you can use the idea. On the 2019 exam, Q2 had two blocks connected by a string over a pulley (a modified Atwood setup), and the fastest path to the system's acceleration was treating both blocks as one system so the string tension dropped out as an internal force. Multiple-choice stems do the same thing, asking why two connected objects share one acceleration or why momentum is conserved in a collision. In FRQ justifications, the magic sentence is some version of "tension is internal to the two-block system, so it does not affect the system's acceleration" or "the collision forces are internal, so the net external force is zero and momentum is conserved." Graders reward that explicit reasoning.
External forces come from outside your system boundary and are the only forces that can accelerate the system's center of mass or change its total momentum. Internal forces act between parts inside the boundary and always cancel in third-law pairs. The trap is thinking a force is permanently one or the other. It's not a property of the force, it's a property of where you drew the system. Tension between two blocks is external when you analyze one block and internal when you analyze both together.
Internal forces are forces between objects inside the same system, and whether a force is internal depends on how you define the system.
Internal forces always come in Newton's third law pairs that cancel, so they cannot accelerate the system's center of mass or change its total momentum.
When two objects are connected by a string and you treat them as one system, the tension becomes internal and disappears from Newton's second law for the system.
Momentum is conserved in collisions because the collision forces are internal to the two-object system, leaving zero net external force.
Internal forces can still deform a system or move parts relative to each other; they just can't move the system as a whole.
On FRQs, explicitly naming a force as internal to your chosen system is a graded justification, not just a shortcut.
Internal forces are forces that parts of a system exert on each other, like the tension in a string connecting two blocks when both blocks are in your system. Because they form equal-and-opposite Newton's third law pairs inside the system, they cancel and can't accelerate the system as a whole.
No. Internal forces cancel in third-law pairs, so they can never change the system's total momentum or the motion of its center of mass. That's exactly why momentum is conserved in collisions when both objects are treated as one system.
External forces come from outside the system boundary and are the only forces that appear in Newton's second law for the system. The same physical force can be either one. Tension is external if you analyze one block alone but internal if you analyze both connected blocks together.
If you treat both blocks and the string as a single system, the string pulls equally and oppositely on the two blocks, making tension an internal force that cancels out. That lets you find the acceleration from just the external forces (gravity) divided by the total mass, which is the strategy the 2019 FRQ Q2 setup rewards.
Not quite. Internal forces can deform a system, transfer energy between its parts, and change how parts move relative to each other (think compression, tension, or a spring inside the system). What they can't do is change the velocity of the system's center of mass.