OK, a relative motion.
Let's explain relative motion.
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
this is block B, OK?
And they both have a velocity.
And so let's say they're moving in the same direction.
Let's say Block A is moving at a velocity of 100 kilometres per hour.
Not be the same is moving at 120 kilometers per hour.
OK, so first of all, let's say these are two vehicles seen from the top climbing on the road with
two lines.
So let's draw the lines like this, casing the road from the top two different lanes and they start
off at the same point to here again and they will be travelling in this direction.
So this one obviously moves faster than this one.
So if you're an observer standing by the side of the road, you will see that.
Vehicle B overtakes vehicle A..
So it'll be faster than vehicle.
But let's say you are in vehicle A. And you observe vehicle B, then vehicle B will move at a relative
velocity to vehicle A..
And that relative velocity will be less than the velocity someone standing by the side of the road will
measure for B..
OK, so how do we find the relative velocity?
We write it down like this.
So you say velocity B relative to A slow velocity B and then you add a slash there and you write A,
the first letter is the velocity of the object we are writing down and the second letter after the slash
is relative to what?
So this would be velocity of B relative to Ali.
OK.
And how you find that is you always subtract something and what you subtract is the velocity of the
object of the first letter.
So that'll be the velocity of B minus the velocity of A, OK.
In this case that we have here, it would be 120 kilometers per hour.
OK, minus a hundred kilometers per hour.
That is 20 kilometers per hour.
So relative to a vehicle, B moves at 20 kilometers per hour, that is the velocity that an observer
sitting and moving along in a vehicle.
I will observe vehicle B moving now vehicle.
I also has a velocity relative to vehicle B, so the velocity of A relative to B would be velocity of
A minus velocity of B, that would be 100 kilometers per hour, minus 120 kilometers per hour.
That is minus 20 kilometers per hour.
So that means if you sit in vehicle B and you observe vehicle eight, look as if the vehicle moves backwards
at the velocity of 20 kilometers per hour.
That is we the minus on there, of course.
And that makes sense if you imagine this situation in your mind.
Now, with these two equations, this one and this one, we can manipulate them and we can say that
if we have the velocity of a relative to be and we have the velocity of B, we can find the velocity
of A mean that would just be, you can say the a velocity of vehicle A would be the A, B, the velocity
of A relative to B plus the B just taken that the B over to the other side.
So over here we have an equation for the.
And we have to find that equation, the relative velocity and velocity of B, you can do the same with
this equation of here.
You can say that the B is the or B relative to A plus the velocity of A..
OK, so those are equations that we'll use in some of the problems.
Now, in this example, it was quite easy because our objects, our vehicles move along a straight line
next to each other relatively.
But let's say you have two objects that move in curved paths relative to each other, something like
this.
Yes, so what we have is we've got a political and political B and political B moves along this path
of we're in that direction, political aims along this path, in that direction.
And as you can see, we have a coordinated access.
We only consider a two dimensional case here.
So we have the X coordinate and a Y coordinate, let's say.
And this particle, as it moves along this path, it will move in each direction, but also in the Y
direction as it goes up the and the same with B.
So in order to find the relative velocity of these two, we need to define these position vectors.
And then from that we can find the relative velocity and also the relative acceleration.