Physics – Newton’s Second Law of Motion


Do you remember the Newton’s first law of motion? It says that an object will continue to do what it’s doing unless acted upon by an unbalanced force or some net force! Now the important question is what happens when there is net force acting on an object? That’s precisely what the second law of motion will discuss. Do you remember understanding the first law of motion with a simple diagram? If the forces are balanced the acceleration will be zero. In the first case, the object will continue to remain at rest. And in the second, the object will continue to stay in motion at the same velocity and in the same direction. Don’t forget. These two cases will remain true only if the forces are balanced. Only if the net force is zero. Now let’s try to create a similar diagram for the second case. If the forces are unbalanced, then there will be acceleration. We saw that in the previous video. And now we come to the interesting part! Let me ask you a question, and I’m sure you can answer it. On which two factors do you think the acceleration will depend on? I want you to tax your brain a bit. Let me repeat my question. On which two factors do you think will the acceleration depend on? Okay, let’s say there is an object on a table. You apply a net force of 10 units on it. And then you apply a net force of 20 units on it . In which case do you think the acceleration will be more? Of course the one in which the force was more. So we can say that one of the things the acceleration depends on is net force. If it’s more, then more will be the acceleration. If it’s less then lesser will be the acceleration. Acceleration depends directly on the net force. Come on, now i want you to think about the second quantity the acceleration will depend on. Let’s say there are two objects on the table. One has a mass of three kilograms, and the other has a mass of five kilograms. Now if you apply net force of ten units to each of them, which one do you think will accelerate more? Surely, it’s the three Kg one because it has lesser mass. Lesser the mass more the acceleration. And more the mass lesser the acceleration. Hence we can say, that acceleration depends inversely on the object’s mass. If you understand this concept well then understanding the second law of motion will be very easy. Mathematically, it can be written like this. In simple terms, the acceleration is directly proportional to the net force applied, and inversely proportional to the mass of the object. That’s the second law. Or as more people understand it like this. The net Force is the product of Mass and acceleration. With this can you tell me the units of force? That should be easy. The standard unit of mass is kilograms. And that of acceleration is meters per second squared. So the unit of force is, kilograms times meters per second squared. And this is what we call as Newton. One Newton is one kilogram times meters per second squared. So if the mass of the object is ten kilograms, and the acceleration is 10 meters per second squared, then the force will equal 100 Newtons. We can say that one Newton is defined as the amount of force required, to give a 1 kilogram mass and acceleration of 1 meter per second Squared.

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