Physics
Physics
Problem
Two identical ice pucks A and B collide on nearly frictionless horizontal ice. The system is defined as consisting of both pucks only. Which of the following forces acting during the collision is external to this system? a) The force of puck A on puck B b) The gravitational force exerted by Earth on puck A c) The gravitational force between the two pucks d) None; there are no external forces acting during the collisionIntuitive explanation
The key is recognizing that external forces are those applied from outside the system boundary. Since your system contains only the two pucks, any force exerted by something other than puck A or B—like the ice surface or Earth—counts as external. Internal forces are the collision forces the pucks exert on each other. Think about what's touching the pucks from beyond the system: that's where external forces come from.
Intuitive explanation
The key is recognizing that external forces are those applied from outside the system boundary. Since your system contains only the two pucks, any force exerted by something other than puck A or B—like the ice surface or Earth—counts as external. Internal forces are the collision forces the pucks exert on each other. Think about what's touching the pucks from beyond the system: that's where external forces come from.