A bullet of mass 20g has an initial speed of 1ms⁻¹, just before it starts penetrating a mud wall of thickness 20 cm. if the wall offers a mean resistance of 2.5×10⁻² N, the speed of the bullet after emerging from the other side of the wall is close to:

a) 0.4 m/s

b) 0.1 m/s

c) 0.3 m/s

d) 0.7 m/s

Answer: d) 0.7 m/s

Solution:

mass, m = 20 g

Initial speed, u = 1m/s

The thickness of wall, s = 20 x 10^-2 m

From the third equation of motion

v² = u² + 2as

Force, F = ma

a = F/m

= -(2.5 x 10^-2)/(20 x 10^-3)

= -1.25 m/s²

v² = 1 – 2(1.25)(20 x 10^-2) = 0.5 = 1/2

v = 1/√2

= 0.7 m/s