# Homework Help: Writing Vector Equations for positions of toy cars in the form r=a+tb ?

1. Jan 18, 2012

### Dosirak

1. The problem statement, all variables and given/known data

For the following remote controlled toy cars, write vector equations for their positions in the form r=a+tb (a=initial position vector, b=velocity vector, r= position vector at any time t sec, distances in cm)

d. the car is travelling at 15cm/s in the direction 3i+4j ad passing through the point (1,-4) at the moment that t=1 second.

2. Relevant equations
[x,y]=(x0,y0)+ t[m1,m2] (?relevant?)
ka=v
|a|=$\sqrt{a^2+b^2}$

3. The attempt at a solution

ka=v
|a|=$\sqrt{3^2+4^2}$
|a|=$\sqrt{9+16}$
|a|=$\sqrt{25}$
|a|=$\sqrt{5}$

... I don't know I think the time is throwing me off

2. Jan 18, 2012

### Delphi51

The speed b is 15 in the direction 3i + 4j.
That is, |b| = 15.
And b = k(3i + 4j).
Take the magnitude of both sides and see if you can solve for k.
It is very much like you wrote in your attempt, but there you said the square root of 25 was the square root of 5 - drop the square root sign in the last step.