Consider an atom in the excited state radiating a photon and goes to the lower energy state. But photons have a certain angular momentum, the momentum itself is not defined. In this case, will there be a recoil of the atom due to photon emitted?

When an object is experiencing free fall, it has a constant acceleration and hence an increasing velocity (neglecting friction). Thus its momentum is increasing. But according to law of conservation of momentum, shouldn't there be a corresponding decrease in momentum somewhere else ? Where is it ?

I read that the relation between temperature and kinetic energy of an ideal gas is only applicable to a large number of particles so that the mean value for kinetic energy has to be considered, so the temperature concept makes sense only as a statistical quantity. But why is that? Using the afor...

I've read 4 different books and yet nobody explains why forces $F_1$ ($=p_1A_1$) and $F_2$ ($=p_2A_2$) point in different directions. Shouldn't $F_2$ point in the same direction as $v_2$? Since we're assuming that parts of fluid between $a$ and $b$ have the same kinetic and potential energies...

I know that macroscopic temperature is a measure of kinetic energy of particles at very low scales (let's call it microscopic kinetic energy). But how can we derive which part of this microscopic kinetic energy gives rise to temperature, and which part instead gives rise to macroscopic kinetic e...

I want to understand how exactly $$ \nabla^2 V = - \frac{\rho}{\epsilon_0}$$ turns into $$ \nabla^2 V = 0.$$ Of course it is by setting $ \rho$ equal to $0$ but what does setting $ \rho$ equal to $0$ mean? $$ \int_{S} \vec E \cdot d \vec a = \int_{V} \nabla \cdot \vec E \, d \tau = \frac {Q}{\...