Stopping Distance Friction Problem

[RebelAngel]

Homework Statement

A 1500 kg automobile travels at a speed of 110 km/h along a straight concrete highway. Faced with an emergency situation, the driver jams on the brakes and slides to a stop.

a) What is his stopping distance on dry pavement (mu = 0.85)?

b) What is his stopping distance on wet pavement (mu = 0.60)?

Homework Equations

My instructor gave me this equation to use to solve the problem:
-µF_N=ma_x

The Attempt at a Solution

I first converted km/h to m/s which came out to be 0.533.
I then plugged my numbers into the formula:
-0.85F_N=1500(0.533)
and came out with this answer:
F_N= -940.6 for a
and
F_N= -1332.5 for b

Im not sure where I messed up, but if anyone could help me that would be great!
Thanks!

 

[Delphi51]

Welcome to PF!

I first converted 110 km/h to m/s which came out to be 0.533.

Write the 110 km/h, then replace the km with 1000 m and the hour with 3600 s, then run it through your calculator. Should be around 30 m/s.
Of course this is the SPEED, not the acceleration!
Use -µF=m*a to find the acceleration. You can find the normal force from the mass of the car.

Finally, after finding the initial velocity and the acceleration, you'll need an accelerated motion formula to find the stopping distance.

 

[RebelAngel]

Ah, now i feel blonde, haha. Thanks so much!

 

[Delphi51]

I've seen it SO many times! It is worth taking a moment to write out that step where you replace the km and the hr. It makes your work reliable and saves all the time you waste catching the mistakes.