The shear force \( F_s \) in orthogonal cutting is given by the relation:
\[
F_s = \frac{F_t \cdot t_c}{t_u},
\]
where:
- \( F_t = 1000 \, \text{N} \) is the cutting force,
- \( t_c = 1.5 \, \text{mm} \) is the chip thickness,
- \( t_u = 0.75 \, \text{mm} \) is the uncut chip thickness.
Substituting the values:
\[
F_s = \frac{1000 \times 1.5}{0.75} = 2000 \, \text{N}.
\]
Thus, the shear force is:
\[
\boxed{625 \, \text{to} \, 750 \, \text{N}}.
\]