Question

A gun of mass 5kg fires 50gm bullet with a velocity of 200m/s. Find the recoils velocity of the gun.

Answer 1

m of bullet = 50g= 50/1000 = 0.05kg
u= 200m/s , v=0m/s
m of gun = 5kg
u= 0m/s  , v= ?
p of bullet before firing = 0.05x200 = 10kgm/s
p of gun before firing = 5x0 = 0kgm/s
Total p before firing = 10 + 0 = 10kgm/s
p of bullet after firing = 0.05x0 = 0kgm/s
let recoil velocity of gun = v m/s
p of gun after firing = 5v kgm/s
total p after firing = 5v + 0 = 5v kgm/s
         According to law of conservation of momentum(p)
        total p before firing = total p after firing
                   10                =        5v
                   10/5             =        v
                     2                =        v
∴ recoi v of gun = - 2 m/s
   negative sign indicates the opp. direction of motion

Answer 2

Recoil velocity = 2m/s