A Levels Physics

A strong wind of speed 33 ms- blows against a wall. The density of the air is 1.2 kgm. The wall
has an area of 12 m? at right angles to the wind velocity. The air has its speed reduced to zero
when it hits the wall.
What is the approximate force exerted by the air on the wall?
A 330N
B 400 N
C 480 N
D 16000N


Choice D is correct because:

Mass (m) of air that hits the wall in one second

Length of air that hits the wall in one second = 33 m

The volume of air that hits the wall in one second (V) = Area of the wall x Length of air that hits the wall in one second

V = 12 m2 x 33 m

V = 396 m3

The density of air (ρ) = Mass of air that hits the wall in one second (m) / Volume of air that hits the wall in one second (V)

ρ = m / V

m = ρ x V

m = 1.2 kgm-3 x 396 m3

m = 475.2 kg

The force exerted by the air on the wall (in one second)

Average force exerted on the air by the wall (F) = Change in the momentum of the air (Δp) / Time taken (Δt)

Change in the momentum of the air (Δp) = Mass of air that hits the wall in one second (m) x Change in velocity of the air as it hits the wall (Δv)

Change in velocity of the air as it hits the wall (Δv) = Final velocity of air (vf) - Initial velocity of air (vi)

Δv = 0.00 m/s - (-33 m/s)

Δv = 33 m/s

F = Δp / Δt

F = (m x Δv) / Δt

F = (475.2 x 33) / 1.0 = 16 000 N

Choices A, B & C are incorrect because they do not correctly state the value of the force exerted by the air on the wall as 16 000 N.

Dynamics | 2013 - M/J - 9702/12 | Q9