If the ratio of all forces acting of the homologous points in model and prototype are equal

• It refers to the similarity of masses and forces of the corresponding particles of flow
• Both geometric and kinematic similarities are pre-requisite for dynamic similarity
• Force is the vector quantities, so that no only ratio of magnitude but also the direction should be parallel

For dynamic similarity

force scale ratio $F_{r}=\frac{(fi)_{p}}{(fi_{m})}=\frac{(fv)_{p}}{(fv)_{m}}=\frac{(fg)_{p}}{(fg)_{m}}$

Where $(fi)_{p},(fv)_{p},(fg)_{p}$=Interia force,viscous forcegravity force at a point in prototype

$(fi)_{m},(fv)_{m},(fg)_{m}$=Interia force viscous force gravity force

at corresponding point in the model