What is the mass of a brick that falls off a cliff and is moving at 20 m/s just before hitting the ground?

the mass of the brick is approximately m ≈ 173 gr

Step-by-step explanation:

If the brick if thrown from a cliff, is likely that it had reached its terminal velocity by the moment it is near the ground. Then by Newton's first law

m*g - 1/2*ρ*C*v²*A = 0

m = 1/2*ρ*C*v²*A/g

where

ρ = density of air. We assume a density of 1.225 kg/m³

v = terminal velocity = 20 m/s

C = drag coefficient = 0.82 for a brick shape

g = gravity = 9.8 m/s²

A = area perpendicular to the ground. We assume that it the smaller area, since is the more stable alignment. Then for a standard brick of 225 mm x 112.5 mm x 75 mm → A = 112.5 mm * 75 mm = 8.43*10⁻³ m²

replacing values

m = 1/2*ρ*C*v²*A/g = 1/2*1.225 kg/m³*0.82 * (20 m/s) ²*8.43*10⁻³ m²/9.8 m/s² = 0.1728 Kg ≈ 173 gr

m ≈ 173 gr