A molecule in a gas container hits a horizontal wall with a speed of `200ms^(-1)` and angle `30^(@)` with the normal and rebounds with the same speed. Is momentum conserved in the collision? Is the collision elastic or inelastic.

Explanation

Let m be the mass of the molecule in kg.
Change in momentum of molecule along the vertical = `2pcos30^(@)`
`=-2m(200)(0.866)=366.4mkgms^(-1)`
Change in momentum of molecule along the horizontal = `(p sintheta)-(p sintheta)=0`
Momentum is conserved along the horizontal.
Since the wall is rigid and momentum change is not observable the net momentum is said to be conserved.
Further, `KE_(i)=KE_(f)` Hence the K.E. of molecules is conserved.