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OK, a relative motion.

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Let's explain relative motion.

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So let's say you've got two objects that could be blocks and let's say this is block A and let's say

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this is block B, OK?

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And they both have a velocity.

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And so let's say they're moving in the same direction.

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Let's say Block A is moving at a velocity of 100 kilometres per hour.

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Not be the same is moving at 120 kilometers per hour.

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OK, so first of all, let's say these are two vehicles seen from the top climbing on the road with

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two lines.

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So let's draw the lines like this, casing the road from the top two different lanes and they start

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off at the same point to here again and they will be travelling in this direction.

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So this one obviously moves faster than this one.

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So if you're an observer standing by the side of the road, you will see that.

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Vehicle B overtakes vehicle A..

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So it'll be faster than vehicle.

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But let's say you are in vehicle A. And you observe vehicle B, then vehicle B will move at a relative

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velocity to vehicle A..

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And that relative velocity will be less than the velocity someone standing by the side of the road will

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measure for B..

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OK, so how do we find the relative velocity?

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We write it down like this.

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So you say velocity B relative to A slow velocity B and then you add a slash there and you write A,

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the first letter is the velocity of the object we are writing down and the second letter after the slash

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is relative to what?

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So this would be velocity of B relative to Ali.

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OK.

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And how you find that is you always subtract something and what you subtract is the velocity of the

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object of the first letter.

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So that'll be the velocity of B minus the velocity of A, OK.

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In this case that we have here, it would be 120 kilometers per hour.

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OK, minus a hundred kilometers per hour.

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That is 20 kilometers per hour.

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So relative to a vehicle, B moves at 20 kilometers per hour, that is the velocity that an observer

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sitting and moving along in a vehicle.

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I will observe vehicle B moving now vehicle.

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I also has a velocity relative to vehicle B, so the velocity of A relative to B would be velocity of

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A minus velocity of B, that would be 100 kilometers per hour, minus 120 kilometers per hour.

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That is minus 20 kilometers per hour.

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So that means if you sit in vehicle B and you observe vehicle eight, look as if the vehicle moves backwards

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at the velocity of 20 kilometers per hour.

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That is we the minus on there, of course.

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And that makes sense if you imagine this situation in your mind.

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Now, with these two equations, this one and this one, we can manipulate them and we can say that

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if we have the velocity of a relative to be and we have the velocity of B, we can find the velocity

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of A mean that would just be, you can say the a velocity of vehicle A would be the A, B, the velocity

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of A relative to B plus the B just taken that the B over to the other side.

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So over here we have an equation for the.

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And we have to find that equation, the relative velocity and velocity of B, you can do the same with

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this equation of here.

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You can say that the B is the or B relative to A plus the velocity of A..

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OK, so those are equations that we'll use in some of the problems.

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Now, in this example, it was quite easy because our objects, our vehicles move along a straight line

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next to each other relatively.

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But let's say you have two objects that move in curved paths relative to each other, something like

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this.

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Yes, so what we have is we've got a political and political B and political B moves along this path

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of we're in that direction, political aims along this path, in that direction.

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And as you can see, we have a coordinated access.

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We only consider a two dimensional case here.

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So we have the X coordinate and a Y coordinate, let's say.

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And this particle, as it moves along this path, it will move in each direction, but also in the Y

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direction as it goes up the and the same with B.

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So in order to find the relative velocity of these two, we need to define these position vectors.

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And then from that we can find the relative velocity and also the relative acceleration.