Imagine that you are hovering next to the space shuttle in earth-orbit and your buddy of equal mass who is moving 4 m/s (with respect to the ship) bumps into you. If she holds onto you, then how fast do the two of you move after the collision?
A question like this involves momentum principles. In any instance in which two objects collide and can be considered isolated from all other net forces, the conservation of momentum principle can be utilized to determine the post-collision velocities of the two objects. Collisions between objects are governed by laws of momentum and energy. When a collision occurs in an isolated system, the total momentum of the system of objects is conserved. Provided that there are no net external forces acting upon the two astronauts, the combined momentum of the two astronauts before the collision equals the combined momentum of the two astronauts after the collision.
The mathematics of this problem is simplified by the fact that before the collision, there is only one object in motion and after the collision both objects have the same velocity. That is to say, a momentum analysis would show that all the momentum was concentrated in the moving astronaut before the collision. And after the collision, all the momentum was the result of a single object (the combination of the two astronauts) moving at an easily predictable velocity. Since there is twice as much mass in motion after the collision, it must be moving with one-half the velocity. Thus, the two astronauts move together with a velocity of 2 m/s after the collision.
For more information on physical descriptions of motion, visit The Physics Classroom Tutorial. Detailed information is available there on the following topics:
Momentum Conservation Principle
Momentum Conservation in Collisions