 Let's solve a couple of questions on normal forces. For the first one, we have a box which is kept touching a wall from one side as shown. Rabindra pushes the box to the right. You can see that against the wall with a force of 120 Newtons. What is the normal force applied by the box on the wall? We need to figure out the magnitude in Newtons and also the direction if it's left or right. As always pause the video and first try this one on your own. Alright, hopefully you have given this a shot. Here we need to figure out the normal force applied by the box on the wall. So let me let me write that in this manner. Normal force on the wall by by the box on the wall by the box. This is what we need to figure out. Normal force on the wall by the box and what all do we know? We know force that is the force being applied on the box. We also know that this box will have some some weight of its own and there is a normal force which will be acting in the upward direction which is which is still acting on the box and this is a normal force on the box on the box by the ground. So I'm writing BG. This is the normal force on the box by the ground. So if we try to draw a free body diagram of the box, we will have some forces. We will have this force to the right. There will be a force to the right, which is the force by Rabindra. There will be a force downwards, which is the weight of the box. There will be a force upwards and this force is this force is the normal force by the ground. Normal force by the ground on the box. So this is box by the ground. And and because the box is being pushed against a wall, there will be a force. There will be a force in the left first direction. This force will be the force by the wall on the box. So I can write this as normal force on the box by the wall by wall. But we need to figure out this force right here. We need to figure out normal force on the wall by the box. Now we can see that the box is not accelerating at all. So all the forces are really balanced. The vertical forces, they are equal to each other. Weight is equal to this normal force and the horizontal forces, they will also be equal to each other because the box is an equilibrium. It's not moving at all. So that means that means that this force, this force F, this force F, this is equal to N, this is equal to the normal force on the box by the wall. And according to Newton's third law, if we think about if we think about this interaction right here, this interaction right here, this side of the box interacting with the wall, there is there is a force by the box on the wall, which is to the right, which will be to the right. And an equal and opposite force is being exerted by the wall on this side of the box. And this is this is this normal force. So they are equal in opposite. So it turns out that the normal force on the box by the wall is equal to the normal force on the wall by the box. They are equal in opposite. This force, this force right here. This one is this force and this one is this force. They are equal in opposite. And because the because the box is an equilibrium, we know that the horizontal forces are balanced. So this normal force is just equal to this force right here, which is 120, 120 Newtons. So the normal force applied by the box on the wall, this is 120. And the direction would be the right direction because the box is pushing the wall to the right and an equal and opposite force is being exerted by the wall on the box, which is on the left. Let's look at one more question. Here we have three blocks, which are placed one above the other on the ground as shown. We have the masses, masses of M, A, M, B and M, C, 3.1, 3.6 and 4. Find the normal force applied by the ground on the block C. We need to figure out the magnitude and the direction. So here the force that we are interested in is this force right here. This is a normal force, which is on block C by the ground. And and how do we know what is the magnitude of this? We can consider these three blocks as one system. We can consider them as one system. We can consider them as one big block and the total mass of this block, total mass of this block, we can consider them as one big block. This is M, A plus M, B plus M, C. So this is really equal to 3.1 plus 3.6 plus 4. And if we add this, this is 6.7 plus 4. This is 10.7 kilograms. So the mass of all of these blocks, that is 10.7 and the force, the force due to the mass of these blocks, this is W, that is equal to Mg. So we can take G as 10. This really comes out to be equal to 107 Newtons. So when we consider these three blocks as one system, there is a force downwards, which is total force 107 Newtons. And this system of three blocks, it is an equilibrium. It's not moving vertically or horizontally. So it means if there is a total downward force of 107 Newtons, there must be a total upward force of 107 Newtons. You can show a free body diagram. You can have these three blocks. This is M, A plus M, B plus M, C. And vertically down, you have three, you have three M, three weights, weights due to these three masses. And because this total, we just calculated, this came out to be 107. There must be an upward force, which is a normal force by the ground to the block, which is really touching the ground. Just C, not on B on A. So this is, this will also be equal to 107 Newtons. So it turns out the normal force on the block C, because it is only block C that is interacting with the ground. That normal force is 107 Newtons and it is acting in the upwards direction. There will be normal forces in between these three blocks, A, B and B and C. But those forces will be internal to this system. So we are not drawing them in this free body diagram. We are considering these three blocks as one system. So therefore we drew 107 Newtons, the total weight due to these three blocks. And because of that, there will be a normal force upwards, that is also 107 because the blocks aren't moving vertically. It's not accelerated, they are not accelerating. So the magnitude of the normal force applied by the ground on the block C, which is touching the ground, that is 107, and the direction is upwards.