# Position Vector Question

1. Oct 8, 2008

### amcqueen

My problem is not so much the mathematics of doing the question, but rather the units.

The question states: "A car travels across a hilly landscape with a position vector given in the x - z plane.

Position Vector = 30 km/hti + 1 km cos (t/(0.1h))k

How can the units for a position vector be given as km/h ?

2. Oct 8, 2008

### Staff: Mentor

The units are not km/h, the units are that of km/h*t = Length/time * time = length.

3. Oct 8, 2008

### amcqueen

ok, i see it now, thanks

4. Oct 3, 2010

### Caldo120

I'm struggling with the maths side of this question. I've done some searching and came across an answer of v(t) = i30 - k10sin10t and I kind of understand this. I do not understand why the sin is negative though. Could anyone run through the differentiation of this?

Thanks!

5. Oct 3, 2010

### Staff: Mentor

You're asking where the minus sign comes in when finding d(cosx)/dx? Have you studied calculus?

Try: http://en.wikipedia.org/wiki/Differentiation_of_trigonometric_functions" [Broken]

Last edited by a moderator: May 5, 2017
6. Oct 3, 2010

### Caldo120

oh dear. apologies. I was flicking through my table of "INTEGRALS" and wondering why it didn't make sense. Thoroughly embarrassed.

Thank you though for showing me I must pay more attention!

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook