# Difference between Streak Line and Path Line

 Difference between Streak Line and Path Line Fig 7.4    Description of a Streak line             Above diagram can be described by the following points:            Describing a Path Line:            a)  Assume P be a fixed point in space through which particles of different identities pass at different times.            b) In an unsteady flow, the velocity vector at P will change with time and hence the particles arriving at P at different times will traverse                different paths like PAQ,  PBR and PCS which represent the path lines of the particle.            Describing a Streak Line:            a) Let at any instant these particles arrive at points Q, R and S.            b) Q, R and S represent the end points of the trajectories of these three particles at the instant.            c) The curve joining the points S, R, Q and the fixed point P will define the streak line at that instant.            d) The fixed point P will also lie on the line, since at any instant, there will be always a particle of some identity at that point.            Above points show the differences.            Similarities:            a) For a steady flow, the velocity vector at any point is invariant with time            b) The path lines of the particles with different identities passing through P at different times will not differ            c) The path line would coincide with one another in a single curve which will indicate the streak line too.            Conclusion: Therefore, in a steady flow, the path lines, streak lines and streamlines are identical.