Portal (video game), Aperture Science Handheld Portal Device

replica movie prop

You appear to understand how a portal affects forward momentum, or to be more precise, how it does not.

Momentum, a function of mass and velocity, is conserved between portals.

In layman's terms: speedy thing goes in, speedy thing comes out.

Also known as a Portal Gun, the ASHPD creates matching color-coded pairs of quantum tunnel portals on surfaces, allowing instantaneous passage between them with conservation of momentum.  A portal gun is needed for Test Subjects to stand any hope of traversing the puzzle mazes and death traps presented by mad supercomputer GLaDOS throughout the vast deserted underground testing chambers of the Aperture Science Enrichment Center.

Full size all-plastic replica. Two points of articulation on each quantum shaping prong. Alternating orange and blue portal mode illumination, toggled by a concealed switch on the grip.  Separate orange and blue portal firing sounds on trigger pull.

From NECA.