A steady force acting over an exposure time delivers an impulse J = F * t, the total momentum transferred to the surface during that window. The impulse is exact only for stated force, exposure time, surface orientation, and absorption assumptions. Real beams add timing jitter, nonuniform profiles, surface losses, alignment, calibration, and measurement uncertainty.
A finite radiation force over time gives exact impulse. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Impulse accumulates over time
The force source and time window determine the impulse.