# Quick Help!

_Mayday_
Question

A ship S is moving with a constant velocity (-2.5i + 6j)Km\h. At time 1200, the position vector of S relative to a fixed origin 0 is (16i + 5j) Km.

a) Find the speed of S

Attempt

No idea, there may be a slight irony in me not being able to do a simple vector question as I also wrote a tutorial on them! I have the velocity and the distance moved, how do I use this to determine the speed? Is the speed not the same as the velocity?

_Mayday_
In this case, is the displacement not the same as the distance? Therefor the answer must be the velocity?

Homework Helper
Gold Member
I think you left something out of the problem, Is it's position vector prior to setting sail given at 12:00 noon, and you want to find it's displacement or position so many hours later? Also, speed and velocity are not the same.

_Mayday_
That is exactly what the question is asking. It is not out of context either, that is what it says and it is th efirst bit of the question. I am aware that speed and velocity are two different things. I was saying that in this case the distance is the same as the displacement would be, so is the velocity not the displacement?

kamerling
The speed is the magnitude of the velocity. The velocity is given. The position is not important for this part.
(and you know only where it is at one moment, so you can't get the speed from that)

Homework Helper
A ship S is moving with a constant velocity (-2.5i + 6j)Km\h. At time 1200, the position vector of S relative to a fixed origin 0 is (16i + 5j) Km.

a) Find the speed of S

Hi Mayday! I assume from the "a)" that there's a b) c) and d) which get progressively more complicated.

I think they're just asking you a really easy question to get you started! Just use Pythagoras! _Mayday_
Yeah, thank you. You see after that the question is a doddle. Thanks.