Remember the experiments with carts described in the previous paragraph. Which forces are internal for a system of bodies consisting of two carts, and which are external?
Internal forces are the forces with which the carts act on each other, while external forces are the forces of gravity and normal reaction forces acting on each carriage. By condition, friction in these experiments can be neglected.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.