Question

A strong wind of speed 33 m s–1 blows against a wall. The density of the air is 1.2 kg m–3. The wall
has an area of 12 m2
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)400N
c)480N
d)16000N

Answer 1

Force is defined as 'rate of change of momentum'

Momentum is calculated as m x v (mass times velocity)

The momentum of 1 kg of air at 33 m/s is therefore 33 kg m/s

After striking the wall its velocity is 0 - therefore its momentum is zero.

You can therefore calculate the change in momentum of this 1 kg of air.

From the surface area and the velocity and the density you can work out the total mass flow rate hitting the wall (i.e. kg/s)

Each of those 1 kg loses the amount of momentum you calculated above, and therefore you can calculate the total 'rate of change of momentum' (kg m/s^2) i.e. the force exerted...