A molecule in a gas container hits a horizontal wall with speed $200 \mathrm{~m} \mathrm{~s}^{-1}$ and angle $30^{\circ}$ with the normal, and rebounds with the same speed. Is momentum conserved in the collision? Is the collision elastic or inelastic?

For an elastic or inelastic collision, momentum is always preserved.

The molecule approaches and rebounds with the same speed of $200 \mathrm{~m} / \mathrm{s}$.

$u=v=200 \mathrm{~m} \mathrm{~s}^{-1}$

Therefore, Initial kinetic energy will be,

$(1 / 2) \mathrm{mu}^{2}=(1 / 2) \mathrm{m}(200)^{2}$

Final kinetic energy will be,

$=(1 / 2) m v^{2}=(1 / 2) m(200)^{2}$

Therefore, kinetic energy is also conserved.