Context : There is still no consensus about progenitor masses of Type IIP supernovae .

Aims : We study a normal Type IIP SN 1999em in detail and compare it to a peculiar Type IIP SN 1987A .

Methods : We computed the hydrodynamic and time-dependent atmosphere models interpreting simultaneously both the photometric and spectroscopic observations .

Results : The bolometric light curve of SN 1999em and the spectral evolution of its H \alpha line are consistent with a presupernova radius of 500 \pm 200 R _ { \sun } , an ejecta mass of 19.0 \pm 1.2 M _ { \sun } , an explosion energy of ( 1.3 \pm 0.1 ) \times 10 ^ { 51 } erg , and a radioactive ^ { 56 } Ni mass of 0.036 \pm 0.009 M _ { \sun } .
A mutual mixing of hydrogen-rich and helium-rich matter in the inner layers of the ejecta guarantees a good fit of the calculated light curve to that observed .
Based on the hydrodynamic models in the vicinity of the optimal model , we derive the approximate relationships between the basic physical and observed parameters .
The hydrodynamic and atmosphere models of SN 1999em are inconsistent with the short distance of 7.85 Mpc to the host galaxy .

Conclusions : We find that the hydrogen recombination in the atmosphere of a normal Type IIP SN 1999em , as well as most likely other Type IIP supernovae at the photospheric epoch , is essentially a time-dependent phenomenon .
It is also shown that in normal Type IIP supernovae the homologous expansion of the ejecta in its atmosphere takes place starting from nearly the third day after the supernova explosion .
A comparison of SN 1999em with SN 1987A reveals two very important results for supernova theory .
First , the comparability of the helium core masses and the explosion energies implies a unique explosion mechanism for these core collapse supernovae .
Second , the optimal model for SN 1999em is characterized by a weaker ^ { 56 } Ni mixing up to \approx 660 km s ^ { -1 } compared to a moderate ^ { 56 } Ni mixing up to \sim 3000 km s ^ { -1 } in SN 1987A , hydrogen being mixed deeply downward to \sim 650 km s ^ { -1 } .