# Homework Help: Acceleration equations involving time.

1. Aug 24, 2011

### cneedshelp

At highway speeds, a certain car is capable of an an acceleration of about 1.7 m/s^2. At this rate, how long does it take to accelerate from 90 km/hr to 110 km/hr? (WATCH UNITS.)

Given- Time=?
A=1.7m/s^2
Vi=90km/hr
Vf=110km/hr

Vf=vi+at Solve for t! T= vf-vi/a 110km/hr-90km/hr = 20km/hr
1.7m/s^2

How do I divide 20km/hr by 1.7 m/s^2?

2. Aug 24, 2011

### bp_psy

First convert your units to be in terms of meters and seconds then do the calculation.

3. Aug 24, 2011

### cneedshelp

I got .02m but i'm not sure how to convert the hr to seconds

4. Aug 24, 2011

### bp_psy

What is that .02m?
You want to convert km/hr to m/s.An hour has 60 mins and a min has 60 sec.
$\frac{1km}{hr}\frac{1hr}{3600s}\frac{1000m}{1km}= \frac {1000m}{3600s}$

5. Aug 24, 2011

### cneedshelp

What I did now was 20(1000)/3600
is 6m/s using sig figs and thats the answer! right?

6. Aug 24, 2011

### bp_psy

Yes if your teacher requires you to round up after every step.I would round after the final answer for time though.