 During a storm, winds blow over a house at 90 km per hour. We can see that in the image over here. The roof, this roof covers an area of 50 m2. What is the upward force on the roof? We are given the air density and we are asked to assume zero speed of the air inside the house. We can also express the answer in Kilo Newtons. Alright, before I work this out, why don't you pause the video and give this one a try? Alright, so let's see what all is given to us. We know the speed, 90 km per hour, we know the area, 50 m2. And there is no speed inside the house. Inside the house, the pressure will also be 180 m at atmospheric pressure. Now, will the pressure be 180 m on the top of the house at the roof, where there is a certain amount of wind speed? If you think about the Pannoli's principle, the pressure above the roof will be less because of the moving wind, right? Which means there will be some pressure gradient. Pressure inside the house, let's say that is P1 and pressure on the top, let's say that is P2. There will be a gradient over here, P1 minus P2. Now, if there is a gradient and we know the area of the roof, the force, the upward force will really be given by the gradient P1 minus P2, multiplied by the area. We need to figure out what is P1 minus P2. So, if we have a look at this image, this is pretty much what we discussed over here. The upward force is because of the pressure gradient P2 minus P, okay? So, in this image, you have P2. Let's correct that. Let's say this is P2, this is P1 and the gradient would be P2 minus P1. So, this right here, assuming P2. This is here, assuming P2. This is P1. So, P2 minus P1. All right. Now, this is the force P2 minus P1 into A. Now, using the Bernoulli's principle, we can try to figure out the pressure gradient. So, before that, let's first write the Bernoulli's equation. That is P1 plus half rho V1 square plus rho g H1. This is equal to P2 plus half rho V2 square plus rho g H2. We can assume H1 to be equal to H2 because we are just really talking about the roof. Right below the roof, you will have H2 right above the roof. You will have H1. So, there is almost no amount of height difference really. We can assume they are equal, which makes our life simpler. We can just cancel them right away. We need to figure out P2 minus P1. So, let's take P1 on the right-hand side. That will give us half rho V1 square minus V2 square equals to P2 minus P1. The other good thing is there is no speed inside the house. So, V2, V2 is really zero. So, P2 minus P1 is nothing but half rho V1 square. And if we substitute half rho V1 square over here, this will be half rho V1 square into area. Now, we know all the values. We know rho, that is 1.3. We know V1 square, that will be 90 square. And we know the area, that is 50. Why don't you plug in the values? Plug in all the values and try to figure out the force. All right, hopefully you did. So, if you calculated it correctly, you should get F as 20,312.5 Newtons. We need to figure out in kilo Newtons. So, we can just divide this by 1000. When we divide this number by 1000, we will get 20.3 kilo Newtons. 20.3 kilo Newtons. We have just divided this value by 1000. And we get the answer in kilo Newtons.