Forces

0:00- [Narrator] You probably heard of the term force. We use it in our everyday life, right? But what exactly is it and what are the different kinds of forces and how do we model them? Well, let's find out. First of all, we think of force as a push or a pull, okay?

0:13And usually in our day-to-day life, we might measure it in pounds, but in physics, we measure it in something called the newtons. And the symbol we use is capital N for that. How much is a newton of force you ask? Well, to get a feeling for it, just hold an apple in your hand.

0:28The amount of force that the apple exerts on your hand, the heaviness that you feel over here, that roughly amounts to a newton of force. But how many types of forces are there and how do we model these forces? Well, let's see. A common force that you might be familiar with is the applied force, like the force that you'd probably apply on a refrigerator to move it across the hall.

0:49But how do you model this? Well, we just draw an arrow mark, since she's pushing this refrigerator to the right, we draw the arrow mark to the right, and the length of the arrow mark represents how strong the force is. If she's pushing harder, if she's exerting a bigger force, we'll put a longer arrow mark.

1:06If she's exerting a smaller force, we'll use a smaller arrow mark. But guess what? The term applied force is a little arbitrary. A more concrete term over here is contact force. We call this a contact force because this is literally a force when two surfaces are in contact with each other, like the surface of your hand and that of the refrigerator.

1:27Another example of this is, let's say when you're throwing a ball, well, your hand is exerting a force on the ball due to the contact. When that contact is lost, that force disappears. Okay? So what are the different kinds of contact forces that we have? Well, think about the floor.

1:43The floor is pushing up on the refrigerator. So it's exerting an upward force on the refrigerator, making sure the refrigerator does not fall through. We call this the normal force. It's normal because this force is perpendicular to the surface. In math, perpendicular is also termed normal.

2:03And if you think about it, even this force is a normal force, because if you think about it, look, this force is perpendicular to the surface. But there's another kind of contact force, which is parallel to the surface. When you're trying to push this refrigerator and make it slide to the right, you can feel some resistance overhead, right?

2:22That resistance force in the opposite direction of which it's sliding or tends to slide, we call it friction. And notice this is also a contact force because it arises due to the contact, but it's parallel to the surface. And by the way, I could have drawn that frictional arrow mark anywhere I wanted.

2:41I just drew it over here so that I can just, you know, write friction on top of it. You're free to move these arrow marks wherever you want, okay? There are also other types of contact forces, but for the sake of this video, we'll not worry about that. So that brings us now to the non-contact forces.

2:54Well, these are the forces that are exerted without being in contact. And a famous example of that is gravity. You probably know that earth is pulling you and me and everything and making it stick to each other. This is also the reason why an apple falls down.

3:08But think about it, the surface of the earth, which is the ground, is not in contact with the apple, and yet it can exert a force downwards, a force towards it. So gravity is a non-contact force, but guess what? Earth isn't the only thing exerting the force of gravity.

3:24In fact, any two masses will exert a force of gravity on each other. So on a larger scale, this is the reason why planets are orbiting the sun, because the sun is exerting a force of gravity on all these planets. Okay. Are there other non-contact forces? Yes. Think about electric force.

3:42For example, if you rub a balloon with your hair and then you bring it near some tiny pieces of paper, look, the pieces of paper starts jumping towards it. Clearly, there's a non-contact force over here. This is called the electric force. It's the same electric force that is running through your computers and lights up or bulbs and any electrical thing that you can think of.

4:06Well, that's due to the electric force. And another non-contact force is the magnetic force. I mean, if you take a compass and you bring a bar magnet close to it, look, the bar magnet can exert a force on this compass without being in contact with it. So magnetic force is also an example of a non-contact force.

4:27In fact, it turns out that even though these two forces look very different, they're fundamentally related and they're actually two different aspects of the same underlying force, which we call the electromagnetic force. The final question for us is what if there are two or more forces acting on a body?

4:46What do we do in such a case? Well, let's see. If the forces are acting in the same direction, for example, you know, here the box is being pushed in the same direction with a force of 10 newtons and 14 newtons, then their effects add up. And so the total force will now be just the sum of the two forces.

5:02So in this case, it'll be 24. So we will say that the total force, or a technical term for that is net force. Net force also means total force, okay? Net force is 24 newtons to the right, because both these forces are to the right. So if there are two or more forces that are in the same direction, just add them up.

5:20But what if two forces are in the opposite direction? Well then the effects tend to cancel each other out. And so now you subtract them. So in this particular case, the total force or the net force would be 14 minus 10 would be four . So net force would be four newtons to the left because the leftward force wins because it's bigger.

5:42What if the two forces had exactly the same strength if both were 10 newtons/ well, then if you subtract them, you would get zero. Then net force would have been zero. And that makes sense. If two people are pushing the same box in the opposite direction with equal forces, not gonna expect that box to go anywhere, right?

5:59So the net force in that case would be zero. Finally, finally, what if along with the horizontal forces, we also had some vertical forces. Now what do we do? Well, now we look at the horizontal and vertical separately. So for example, in the horizontal, we already know the net force is four newtons to the left, and now we look at vertical separately.

6:21So in the vertical, look, both forces are exactly equal and opposite. So when you subtract them, 100 minus 100, you get zero. So we would say, hey, the net force is zero. All right, long story short, a force is a push or a pull, and it's measured in newtons. Whenever a force arises due to the contact between surfaces, we call them contact forces.

6:41The two common ones are the normal force where the force is perpendicular to the surface and the frictional force, which is a force that's parallel to the surface. And you can have non-contact forces like gravity or electric force or magnetic forces. Here, the forces are exerted without having a contact.

7:00And if there are two or more forces acting, well then if they're in the same direction, we add them up. If they're in the opposite direction, we subtract them. And finally, if there are forces both in the horizontal and the vertical, we analyze them separately.