* Stress is defined as force per unit area (Force/Area), making its unit N/m² (Pascal).

* Strain is a measure of deformation, defined as the change in length divided by the original length. Since both length and change in length have the same units (e.g., meters), strain is a ratio of two lengths, making it dimensionless.

* Momentum is mass times velocity (mass * velocity), resulting in units of kg*m/s.

* Force is mass times acceleration (mass * acceleration), giving it units of kg*m/s².

Therefore, only strain is dimensionless.