NCERT Solutions for Class 9 Science Chapter 9

Force and Laws of Motion

NCERT Books for Session 2020-2021

CBSE Board and UP Board

Intext Questions

Page No-126

Questions No-3

# 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.

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Mass of the rifle, m1 = 4 kg

Mass of the bullet, m2 = 50g = 0.05 kg

Recoil velocity of the rifle = v₁

Bullet is fired with an initial velocity, v₂ = 35 m/s

Initially, the rifle is at rest.

Thus, its initial velocity, v = 0

Total initial momentum of the rifle and bullet system = (m1 + m2) v = 0

Total momentum of the rifle and bullet system after firing:

= m₁ v₁ + m2v2 = 4(v1) + 0.05×35 = 4v1 + 1.75

According to the law of conservation of momentum:

Total momentum after the firing = Total momentum before the firing

4 v₁ + 1.75 = 0

⇒ v₁ = – 1.75/4 = – 0.4375 m/s

The negative sign indicates that the rifle recoils backwards with a velocity of 0.4375 m/s.

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