 Let's solve a couple of questions on analyzing motion using Newton's laws. For the first one we have a block which is kept between Bhopain and Pranab. Bhopain pulls it towards the left and Pranab pulls it towards the right. Initially Pranab applies a larger force and the block starts moving toward him i.e. in the right direction. But soon after Bhopain increases his force to match Pranab's. The net force on the block thus becomes zero. We need to describe the motion of the block immediately after the net force on it becomes zero. Alright, why don't you pause the video and give this one a try first. Alright, hopefully we have given this a shot. Now let's try to represent what's happening. So we have a box and we have two people, Bhopain who is pulling the box towards the left and Pranab who is pulling the box towards the right. So what happens initially, Pranab applies a larger force. This represents a larger force with a longer arrow and Bhopain is applying a smaller force initially so a smaller arrow. So because of this what happens the block starts moving toward right i.e. towards Pranab and we can see that why that is happening. So if this is the force applied by Pranab, if this is the force applied by Bhopain initially we know that the condition is Fp is greater than Fb which means that there will be a net force in the right direction. Let's show the net force with the blue arrow. So there is this much of net force, this is F net. And whenever there is an F net we know that there will be an acceleration involved. This is the Newton's second law and in the same direction as the force. So we have the box moving to the right with some acceleration. But soon after Bhopain increases his force to match Pranab's. So now what happens, the force, this force right here, this is no longer lesser than the force applied by Pranab's but it is exactly the same. So now you can see what will happen, there will be no net force, right? These two forces are the same, that is the second case. These two forces are the same. So F net is really zero. There is no F net, F net is just zero. So we might think that when F net becomes zero the block stops moving. But the block was already moving to the right and then F net, the net force becomes zero. So would the block really stop moving? Let's try and understand that by bringing in some numbers. So let's go back to the initial state. That was when there was an F net and there was no Fb equal to Fb but in fact the force due to Pranab was more than the force due to Bhopain. So in this when the condition was like this, I'm drawing it over here, then there was a net force, right? The net force was towards the right. And when there is a net force there will be some acceleration. Let's say assume a random number, let's say the acceleration is over here, 10 meters per second square, completely not random number. But now when Bhopain started increasing the force to the left when this arrow became even longer, at this point there was some net force, not as much as the initial case but still there was some, right? Which means there will still be some acceleration but much less than 10, maybe 4, we don't know, 4 meters per second square. The block is still moving to the right with increasing velocity because there is a positive acceleration. Now when Bhopain further increased the force to the left so that the force to the right and the force to the left they were equal to each other, there was then no net force. Net force was 0. Which means acceleration is 0. But the block was already moving to the right because it experienced 10 meters per second acceleration square, then it might be experiencing more acceleration, 6, 7, 8 and 4 to the right. So it is experiencing all these positive accelerations to the right which means the velocity was continuously increasing and it was moving to the right. And now when the acceleration became 0, velocity doesn't really become 0 but the velocity stops increasing and it already had some velocity because it experienced so many accelerations. So the block keeps on moving to the right and that is what should be, the block continues moving to the right with a constant, constant velocity. Alright, let's look at one more question. Here we have an object which is moving to the right with a certain velocity. It starts slowing down at time t1 and comes to rest at time t2. It stays at rest thereafter. Which of these could correctly describe the variation of the net force on the object from t1 to t2. Again pause the video, read the options, give this one a try first. Alright, hopefully you have given this a shot as well. So again let's try to represent what's happening. So we have an object, let's say here is the object and the object is moving to the right with some velocity v. Now at time t1, let's say this is time t1, the object starts slowing down, it starts slowing down and at time t2 the object comes to a state of rest. Here the velocity from v it starts to slow down, starts to slow down, slow down and then velocity becomes 0. Which of these would correctly describe the variation of the net force on the object from t1 to t2. So let's think about it. If the velocity is decreasing, if velocity decreases, then it means there must be an acceleration opposite to the direction of motion. So that means if for this case we have drawn the block to the right, moving to the right, so acceleration would be to the left, there will be an acceleration to the left. And whenever there is an acceleration involved, whenever there is an acceleration, we know there will be some force, right? Because forces cause accelerations and according to Newton's second law, you have f net which is equal to ma force. If there is an acceleration, there will be a force involved. So that means there must be a force to the left direction. We don't know what that is. There could be friction, could be air resistance, could be someone else pushing the block in the opposite direction. That's not relevant. We need to think about the variation of the net force. So if the block starts slowing down, there must be a net force which will be to the left. Okay, so in the question it's already given that the block is moving to the right. So we drew it also to the right, that's good. So the force would be to the left, could be option b and could be option c. Now option b says net force is toward the left and constant, net force is toward the left and decreasing. Okay, now the question is, is the force constant or is the force decreasing? Let's read the question again. We have object moving to the right with a certain velocity, alright? Starts slowing down, comes to rest and it stays at rest thereafter. So at t2, the block comes to a state of rest, there is no velocity, neither moves to the right nor to the left. Now if there would have been a force, if there would have been a constant force toward the left, then after coming to a state of rest, the block would start moving to the left because the force is to the left direction. But if the force is decreasing, if the force was this much to begin with, then decrease slightly, then decrease even more, then even more, then became zero. Then the block will come to a state of rest and it will stay at rest because now there is no force which can cause an acceleration towards the left. So the force must be decreasing because the block stays at rest thereafter. If the block started moving to the left, then the force would have been a constant force, right? Which would have caused an acceleration, which would have moved the block in the left direction. But the block is at rest, which means there is no acceleration and which means there is no force. So this one is option C. Alright, you can try more questions from this exercise in the lesson and if you're watching on YouTube, do check out the exercise link which is added in the description.