False. The path of one particle as seen by the other in projectile motion is not always a straight line. Projectile motion refers to the motion of an object that is launched into the air and moves along a curved path under the influence of gravity. The path of a projectile is typically parabolic, forming an arc.
If two particles are both in projectile motion, their paths would generally be parabolic and not straight lines. The specific shape of the paths depends on the initial conditions and the forces acting on the particles. However, if one particle is stationary or moving at a constant velocity, then the path of the other particle as seen from the reference frame of the stationary or constant velocity particle would indeed appear as a straight line.