Calculating Final Velocity of a Car Using Uniform Acceleration

[CTech2015]

Homework Statement

A car enters the freeway with a speed of 6.4 m/s and accelerates uniformly for 3.2 km in 3.5 minutes. How fast in m/s is the car moving after this time?

Homework Equations

DeltaX=1/2(vi+vf)(DeltaT)
Displacement=1/2(initial velocity + final velocity)(Time interval)

The Attempt at a Solution

3.2=1/2(6.4+x)(3.5)
x=-4.6
? Why is this negative? What is the final velocity?

 

[Nathanael]

It is important to be aware of which units you are using!

 

[CTech2015]

What do you mean? Do we need to convert the km to m first?

 

[Nathanael]

You need to convert everything into the same units, It only makes sense to add/subtract/cancel units which are the same.

For example, if I'm traveling 50 miles per hour, in 1 second will I have traveled 50*1=50 miles?
Of course not, because seconds are not the same as hours. (This is essentially what you did wrong.)