When two objects are in contact and move together (i.e., they share the same motion), the following statements must be true:

  1. They exert the same magnitude force on each other.
    • By Newton’s third law, any interaction between two objects involves equal and opposite forces.
    • So, if object A pushes on object B, object B pushes back on object A with the same magnitude force in the opposite direction.
  2. They have the same acceleration.
    • If the objects are “moving together” (for example, a box resting on a cart and both speeding up along the floor), their positions relative to each other do not change.
    • That only happens if their accelerations are equal; otherwise, one would pull ahead or lag behind.
  3. What is not necessarily true:
    • They do not need to have the same net force on them.
      • Net force depends on both interaction forces and all other forces (like gravity, normal force, friction).
      • If their masses differ, equal acceleration requires different net forces Fnet=maF_{\text{net}}=maFnet​=ma.
    • They do not need to have the same weight.
      • Weight depends on mass and gravity; two contacting objects can certainly have different masses and therefore different weights.

So, from a typical multiple-choice list like:

  • The objects must exert the same magnitude force on each other.
  • The objects must have the same net force acting on them.
  • The objects must have the same acceleration.
  • The objects must have the same weight.

The statements that must be true are:

  • “The objects must exert the same magnitude force on each other.”
  • “The objects must have the same acceleration.”