Question

NCERT CLASS IX - Science Textbook
Chapter 9 - Force and Laws of Motion

From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m s - '1. Calculate the initial recoil velocity of the rifle.

Answer 1

we know that [ m1.u1+ m2 u2 = m1.v1 + m2.v2 ]

here initial velocity of the rifle (u1) =0
its mass (m1) = 4kg

mass of the bullet (m2)  = 50g=.05kg
its velocity before firing  (u2) = 0

hence we get, m1u1 + m2u2 =0

the momentum of the bullet and the rifle after firing = m1v1 + m2v2 =0 
i.e. 4kg.v1 +0.05*35 = 0
     4 v1 +1.75 = 0
     4.v1 = -1.75
     v1 = -1.75/ 4 = 0.4375 = 0.44 (approx.)

Answer 2

m1 = 4 kg 
m2 = 0.05 kg 
v2=35m/s
v2 ?
seedha   potential ko conserve kr do 
m1u1 + m2u2 = m1v1+m2v2
initial velocity har ek me zero h to 
m1v1+m2v2 = 0 
4×v1 + 0.05 ×35 = 0 
4v1 =1.75 
v1 =1.75/4
v1 = 0.43 m/s