# Car acceleration problem

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1. Feb 6, 2015

### CTech2015

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

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

3. 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?

2. Feb 6, 2015

### Nathanael

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

3. Feb 6, 2015

### CTech2015

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

4. Feb 6, 2015

### 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 travelled 50*1=50 miles?
Of course not, because seconds are not the same as hours. (This is essentially what you did wrong.)