 going forward with 15 meter per second, then I will make the question, moving with a constant velocity of 15 meter per second, as in what direction should a stone be projected up, the stone which is projected, okay. In order to find out in what direction should you project the stone, you need to find theta, what direction a stone is projected up with a velocity of 30 meter per second, so as to appear to the truck for a person standing on earth. So this person, see this velocity is minus the velocity of the truck, velocity we need to find right, what is that, to the observer who is standing on the ground, observer who is standing on the ground, we will see the absolute velocity, yes or no, so that as a stone right, this is let's say Vs, this is V truck, this should be what, this should be equal to Vs minus V truck in minus of V truck, okay, let's say this is minus of V truck, okay. So now this is the relative velocity which is Vs minus V truck, this I am taking let's say 5, so 10 of 5 is how much, this length divided by that length which is 15, I don't know how much it is right, I know this length i put it as, so I'll say sign of 5 which is 15 by 30, 1 by 2, 5 is 30 degrees and this is 19, what do you think? Yes, it is minus in length, any doubt that comes again and again in relative motion to the tax river boat problem. Suppose the width of the river, the swimmer's capability U and river is, this is velocity, the swimmer's velocity is like this. The swim is a relative velocity. Now swimmer can, okay, both of you stand. Person who is swimming and this is the velocity of river. This is the shortest distance, okay. This is the velocity of the river. Now tell me which direction. Suppose velocity of river is V and swimmer's capability is U. The swimmer should swim so that it is travelling the shortest distance. If a person comes in exam to tell you as this way, you will tell like that. Shortest distance. A person standing on the ground should feel that the person is going like this. So it becomes that way. So if this is swimmer's capability U and this is the V, this angle is how much? Theta, sine of theta should be equal to V by U. Theta is sine inverse of V by U. Understood this? All of you understood? If water is moving, simple, keep it simple, okay. So here is the question of river is flowing with velocity V, flowing with velocity V. Now swimmer, swimmer is there. Move in this straight line. So his relative velocity of river is V distance. How far away he will be? This distance will be water. He will be tested. First when you get the answer, check damage speed. When you get the answer, check damage speed correct now. Damage repetitions and your problem is some velocity, right? I asked you to find your distance. When you find out V, you can find out. swimmer with respect to water, this is the velocity. This is U, this is V. This is how much? U squared plus V squared. Okay. Now listen here. Now I know the distance. I know that distance, this one. Displacement in this direction I know which is V. Do I know velocity? That will be this velocity, cos theta which come up. Yes or no? Along this direction it will be. So time it will take to cross the river. This is the answer V d by U. This is here. That's simple. You are complicated. This distance also, this one. Okay. That will be what? This is this. Will be also same. V only. It will come out to be d by U itself. Displacement from here to there. Direction. Displacement is V only. Even though you have gone there. Okay.