## Astronaut Catch

A GIF Animation

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. Detailed
information is available there on the following topics:

Momentum
Momentum Conservation
Principle

Isolated
Systems

Momentum Conservation
in Collisions

Other animations can be seen at the Multimedia
Physics Studios.

© Tom Henderson, 1996-2007

All Rights Reserved