Intro to forces (part 1)

0:00- [Instructor] A force is just a push or a pull, that's it. But in this video, we're gonna explore the different kinds of pushes and pulls that we'll encounter in our daily lives. So let's start with an example. Imagine you are pulling a chair in your living room using a rope for some reason.

0:16Let's think about all the different kinds of forces we can imagine here. Well, the first one is the applied force. An applied force is basically, as the name suggests, the forces that we apply either by pushing or pulling on things. In this case, we are pulling, but in some cases you can push on things.

0:33So that's the most common one. But besides that, what other forces can we think of? Well, look, although you are the one who's pulling on the rope, but who's pulling on the chair? Well, it's the rope that is pulling on the chair. And this force that ropes and strings put is what we call a tension force, which will represent as FT.

0:54A tension force will always be a pulling force and it'll always be along a rope or a string. And it's called tension force because this force exists, because this rope is tensed, more the rope is tense, more will be this tension force. You can imagine if this rope was slacking, there will be hardly any tension force here.

1:14Okay, what else? Well, then there is a gravitational force. We all know that Earth is pulling us all down with gravity. All masses tend to pull on other masses like you and I and this chair all are pulling on each other, but it's the Earth's gravity that's significant.

1:30And so Earth's gravity will always be a pulling force. And this force is often what we call the weight. But now we can ask, "Well, why isn't the chair just falling down due to gravity?" Ah, because the floor doesn't allow it. The floor pushes back up on the chair.

1:47And this force with which the floor is pushing up on the chair, we call it the normal force. But why is it called normal? Well, it's not normal, abnormal kind of normal. Normal in physics or math means perpendicular. So it's representing perpendicular. And you can see what it means is that this force is perpendicular to the surface.

2:09Let me take another example. If you were to push on, let's say a wall, then the wall will push back and that will also be a normal force. And you can again see that force is perpendicular to the surface. So yeah, we could have just called it the perpendicular force, but we like to call it the normal force.

2:26Normal means perpendicular. And of course, the normal forces are acting at this point of contact. But just to keep this board clean, I'm just representing it over here. But what's important, it's an upward push from the surface. What else? Well, if you have pushed and pulled on furniture, you know that it's extremely hard to get them moving.

2:46Why is that? Again, you might know friction. To understand friction, let's look at the points of contact at a microscopic level. Now, although these things appear smooth at a microscopic level, they're not, you'll have mountains and valleys. And now when this chair, the leg of the chair tries to move past the floor, you can see that there's going to be some resistance.

3:09There's going to be some blockage due to these mountains and valleys, due to the roughness. That is the frictional force. So if the chair tries to move this way, the friction, the resistance for that motion is in this direction. And so that's another force, the frictional force.

3:26It's always going to be a push, and the direction will always oppose the sliding motion. So in this case, the chair tries to slide to the left and therefore, it will oppose it by pushing it to the right. To explore more forces, let's take a few more examples. Can you pause the video now and try to identify all the different kinds of forces that you can think of?

3:45All right, let's see. Well, the first one is, of course, gravity. Gravity is gonna give them all their own weight. But let's come to the rocket and ask ourselves, "Well, what makes the rocket go up?" Well, it's pushing down all these gases. And in doing so, it generates a force called the thrust.

4:03Thrust is a force that you generate when you push something in the opposite direction. So the rockets are pushing the gases down. That generates an upward thrust. The submarine does the same thing. Instead of gases, the submarine pushes the water backwards and it generates a forward thrust.

4:21Same is the case with the airplane. The engines of the airplane push the air backwards and it generates a forward thrust. What else? Well, notice that all these things are moving through a fluid, air or water or liquid in general, they're all fluids. And as things move through the fluids, the fluid atoms and molecules are going to hit them in the opposite direction.

4:45As the rocket goes up, the air molecules are gonna go and hit them down. Same is the case over here. Same is the case over here. The water molecules are gonna go hit them backwards. And that's going to generate a backward force, which we call drag. A drag force is a push that's generated due to the molecules and the atoms of the particles of the fluid hitting these things.

5:09It's always going to be in the opposite direction of the motion. And so in some sense, these forces are kind of similar to friction in the sense that they both oppose motion, but of course, they're very different forces. And drag forces in air are also often called air resistance.

5:27And I'm just like name dropping at this point. But air resistance, just wanna say that air resistance is the same thing as drag force. Drag is a general word, but for drag in air, we often also call it as air resistance. Cool, a couple of more forces. And for that, let's ask ourselves, "What keeps the summary in afloat?

5:45Like what counters gravity? What counters its own weight?" Well, it turns out that when you submerge an object in a fluid, again a liquid or air, the fluid always generates an upward push, or to be more specific, a push against gravity. This push is called the buoyant force.

6:07Buoyant or buoyancy means floating. So it's literally the force that makes things float. But why do fluids do that? Well, think of it this way. If there was no submarine over here, there would be a hole in the shape of the submarine. And immediately, water would try to rush into that hole, right?

6:26But of course, there is no hole. There is a submarine over here so the water can't rush in. Instead, they'll end up pushing on the submarine from all the directions. Now it turns out that if you carefully add up all those pushes, they will all add up to give you a force upwards.

6:41And that's what the buoyant force is. In short, whenever you submerge something in any fluid, there will always be an upward push or push against gravity, which is called the buoyant force. And now you may ask, "Well, shouldn't that be the case here as well in air as well?"

6:57Sure, there will be buoyant forces acting on the rocket and the plane or anything that's in the air because air is also a fluid. But the buoyant force in the air is usually very tiny. And so when it comes to rocket and the airplane, these other forces are so massive that you can completely ignore the buoyant forces.

7:14But sometimes they can be significant. Think about helium balloons. Helium balloons are so light that it's the buoyant force from the air, that's what makes them go up. All right, finally, we can ask ourselves, "Well, what makes the plane go up?" It's not the buoyant force because it's not big enough over here, but there's some other force that gets generated, which we call the lift.

7:36It's called lift because it's literally lifting the plane up. Where does this force come from? You can probably guess it comes from the wings, but what exactly happens? Well, turns out that's a little complicated. It's aerodynamics. But in short, the wings are designed in such a specific way that whenever air flows over it, there's going to be a force that's generated.

7:59Now depending on that design, the force can either be up or down. Of course, we design it in such a way that the force has to be upwards. And like I said, we'll not worry too much about why and how that happens. It's mostly gotta do with how the wings are designed.

8:13But what's important is that the strength of this lift force depends on how fast the wind flows over it. The faster the wind flows, bigger is the lift force. And that's the reason why your airplanes have to maintain a very high speed because you want the wind to be flowing at a very high speed to generate a high enough lift.

8:34And that's the reason why you have huge runways so that the airplanes can take all that time to accelerate to that minimum takeoff speed. And that's pretty much it. Wow. Just look at the rich varieties of pushes and pulls that we have in our everyday life. And there could probably be more.

8:51And so it's very easy to get overwhelmed. But don't worry about it. As you get more practice, you'll get used to it. But if you quickly summarize, we have the applied force, which can be a push and pull. It's usually the forces that we apply in our daily life.

9:03The tension force will always be a pulling force, usually in strings and ropes. You have gravity that gives you weight, always downwards. You have the normal force, which is always going to be a perpendicular force to the surface. It's always going to be a push.

9:16Friction and drag forces try to oppose the motion. You generate thrust by pushing things in the opposite direction. And buoyant forces is an upward push, push against gravity when things are submerged in fluids. And lift force gets generated when you have aerodynamically designed wings.

9:33And these forces depend on the design and of course, but most importantly on the speed at which the wind flows over it. But what truly blows my mind away is that even though we have so many different varieties of forces, the way we model them, the way we analyze them, they're units in everything, that stays the same.

9:51Isn't that beautiful and wholesome? Stay tuned for more on that.