Intro to forces (part 1) | Physics | Khan Academy

A force is just a push or a pull, that's it. But in this video, we're going to explore the different kinds of pushes and pulls that we will 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. But think about all the different kinds.

Well, the first one is an applied force. It 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. So, 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, who's pulling on the chair? Oh, it's the rope that is pulling on the chair. This force that ropes and strings put is what we call a tension force, which we will represent as F_t. A tension force will always be a pulling force and will always be along a rope or a string. It's called a tension force because this force exists due to the rope being tensed. The more the rope is tense, the more will be this tension force. You can imagine if this rope was slacking, there would be hardly any tension force here.

Okay, 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. And 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, 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. This force with which the floor is pushing up on the chair, we call it the normal force. But normal... well, it's not normal; it's a kind of normal. Normal in physics or math means perpendicular. So, it's representing perpendicular. You can see what it means is that this force is perpendicular to the surface.

Let 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. Normal 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 is that 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. Why 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 are 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. There'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. It'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?

Alright, let's see! Well, the first one is of course gravity. Gravity is going to 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 on these gases, and in doing so, it generates a force called the thrust. Thrust 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. Same 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. As the rocket goes up, the air molecules are going to go and hit them down; same is the case over here.

The water molecules are going to 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, or the particles of the fluid hitting these things. It's always going to be in the opposite direction of the motion.

In some sense, these forces are kind of similar to friction in the sense that they both oppose motion. But of course, they are very different forces. Drag forces in air are also often called air resistance. I know I'm just like name dropping at this point, but air resistance...just want to 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 air resistance.

Cool! A couple more forces, and for that, let's ask ourselves, what keeps the submarine afloat? Like, 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.

Buoyant 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? But, of course, there is no hole; there is a submarine over here. So the water can't rush in; instead, they will 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. And 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? Sure! 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 rockets and airplanes, these other forces are so massive that you can completely ignore the buoyant forces. But 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.

Alright! 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. It'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, it turns out that's a little complicated—it's aerodynamics. But, you know, 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. Now, 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 got to do with how the wings are designed. But 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, the bigger is the lift force. And that's the reason why 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.

And 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. There could probably be more, and 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. The tension force will always be a pulling force, usually in strings and ropes. You have gravity that gives your 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. Friction and drag forces try to oppose the motion. You generate thrust by pushing things in the opposite direction. Buoyant forces is an upward push against gravity when things are submerged in fluids. Lift force gets generated when you have aerodynamically designed wings, and 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, their units and everything, that stays the same. Isn't that beautiful and wholesome? Stay tuned for more on that!