 If you drop a basketball and a feather in vacuum, then you will see that they will fall exactly at the same rate. They would hit the ground at exactly the same time. But why? I mean, think about it. The basketball is clearly experiencing a much bigger gravitational force. Earth is pulling on it with a much higher force. That's why it's much more heavy. And the earth is putting a much smaller gravitational force on the feather. That's why it's so light. So even though earth is pulling the basketball with a much larger force compared to the feather, still they both fall at the same rate. Why? Because my brain is thinking that if this is being pulled harder, then it should fall faster, right? Then why are they both falling at the same rate? To answer this question, we need to explore Newton's second law. So let's do that. In simple terms, Newton's second law of motion says that the net force acting on an object will always equals the mass of the object multiplied by the acceleration of the object. And this is the famous F equals ma. Now just from this, we can now identify what the units of force is going to be. You see, the units of force is going to be the unit of mass, which is kilograms, times the unit of acceleration, which is meters per second square. And to honor Newton's work, this unit is also called the Newton. So the standard unit of force is also called a Newton. But remember, a Newton is basically kilogram meters per second square. Now let's say what is this equation really trying to tell us? What does it mean to say F equals ma? Well, remember in Newton's first law, we learned that when there is no net force acting on an object, the object will continue with state of rest or state of uniform motion. From there, we learned that that meant that if there is a net force acting on the object, then the object will accelerate, right? We learned that. And so Newton's second law basically answers how much acceleration you get when you put a net force on it. And so a better way to actually look at this formula is we could say the acceleration that a body gains when a force acts on it is given by this expression. So the acceleration would be the net force acting on a body divided by the mass of the body. So the Newton's second law is actually connecting, it's a connection between acceleration, force and the mass. Now before we understand a little bit more, let's quickly take an example. So imagine you have an object like say a refrigerator which has a mass of 15 kilograms. And now let's say you put a force of 45 Newtons on it. What will be its acceleration? Won't you pause, why don't you pause and give it a try yourself, okay? So the acceleration would be the net force. Here there's only one force that is mentioned over here. So the net force is 45 Newtons divided by 15 kilograms. And that will be three meters per second square. So look, by using Newton's second law, I can calculate the acceleration. Now just from this example, we can learn so many things. For example, you see that if there was more force acting, if this force was more, the acceleration would be higher. That makes sense, right? That's what exactly it's saying. More of the force, more of the acceleration. But what if this refrigerator was more massive, much more heavier and you put the same force? Well then it makes perfect sense, right? The acceleration would be much smaller now, right? And that makes sense. See, this denominator would be much bigger making the acceleration smaller. So it all makes perfect sense. So you can see more of the force, more of the acceleration. But bigger the mass, more the inertia, smaller the acceleration. That makes sense. The second thing that you understand is that the force, this equation, if you look at the equation carefully, it might seem as if, hey, from the equation, it says that the force depends on mass and acceleration. That's not true. Force does not depend on mass and acceleration. The force that you apply on the refrigerator, for example over here, depends on you. It depends on how strong you are, for example. If Earth is applying the force, say, of gravity, then it depends upon how strong gravity is. It has nothing to do with acceleration or mass. Force is an independent variable. Same as the case with mass. Mass is also another independent variable. Mass has something to do with how much amount of stuff it is there. What do you keep inside the refrigerator, for example? That's what the mass depends on. So you see, force and mass are two independent variables. They don't depend on any of these, any of these other two. It's the acceleration that's a dependent variable. It's the acceleration that depends on these two. That's the reason why I like this formula as Newton's second law. That acceleration is net force divided by mass. Because this might make us feel like it's the force that depends on mass and acceleration, but that's not true. It's the acceleration. Newton's second law is telling us about acceleration, what it depends on. It depends upon force and mass. Okay, now that we are clear with this, let's go back to our original question that we asked. Why is it that even though there's a lot of force on this extremely heavy football that Earth is putting on it compared to this feather, still they're both falling at the same rate. Why? Can you now answer this question? Can you pause and think about it? The answer is actually in the question. See, although there is more force acting on the ball, the ball is also much more massive. It has more inertia. We just saw that acceleration not only depends on the force, but also depends upon inertia. More force, but much higher inertia, much higher resistance to change. But on the other hand, what about the feather? A feather might be having a much smaller force, but it also has a much smaller mass. It also has a much smaller inertia. So you see on one hand, you have an object which has a much bigger inertia, very hard to change the resistance, very hard to change its state, and there is more force, and you have an object which has a much smaller mass, and there is a small force. And it turns out if you divide the two, you will get the exact same result. They both compensate, and therefore the acceleration that they both achieved due to gravity is the same, and therefore they fall and hit the ground together at the same time. Beautiful, isn't it?