External force
Also called Net external force
An external force is one exerted on a system by something outside the system boundary. Whether a given force counts as external depends entirely on where you drew that boundary, and every conservation of momentum argument turns on the choice.
Draw the boundary first. Forces crossing it are external, forces between objects inside it are internal, and the two do different things.
AP Physics 1 EK 2.3.A.2 disposes of the internal ones: interactions between objects within a system, the internal forces, do not influence the motion of a system's center of mass. They come in Newton's third law pairs with both members inside the boundary, so they cancel.
What is left drives everything the system does as a whole:
- EK 4.2.A.1: the rate of change of momentum equals the net external force exerted on an object or system.
- EK 4.3.A.1.ii: the velocity of a system's center of mass is constant in the absence of a net external force.
- EK 4.3.B.2: if the net external force on the selected system is zero, its total momentum is constant.
Same collision, two answers, both right. Take one cart as the system and the other cart's push is external, so that cart's momentum changes. Take both carts and the identical push is internal, so the total momentum holds steady. Nothing physical changed; you changed which forces you had to count.
It is also why momentum is treated as conserved through a collision with gravity and friction present. EK 4.1.A.3.i defines a collision as an interaction where the forces between the involved objects are much larger than the net external force during the interaction, so the external ones contribute almost no impulse over that interval.
The conservation of momentum guide works the routine; isolated system covers the boundary conditions each conservation law needs.