Assume your file cabinet is sitting on a cart with wheels.  It is sitting in the middle of a room with a smooth floor.  You want to move it against the wall.  You give it a push.  It takes off, and before you know it, it slams into the wall.  It is hard to stop it, because it has linear momentum.

Linear momentum is a measure of an object's translational motion.  The linear momentum p of an object is defined as the product of the object's mass m times its velocity v.

p = mv.

Linear momentum is a vector.  Its direction is the direction of the velocity.  The Cartesian components of p are

p_x = mv_x,  p_y = mv_y,  p_z = mv_z.

The SI unit for the magnitude of the momentum p is kg m/s.

Conservation of momentum

In interactions between isolated objects momentum is always conserved.

If two isolated objects collide, we have
m₁v_1i + m₂v_2i = m₁v_1f + m₂v_2f.