Car's velocity after being hit by a force

[KatieLynn]

Homework Statement

A 2500 kg car traveling to the north is slowed down uniformly from an initial velocity of 25 m/s by a 6240N braking force acting opposite the car's motion. What is the car's velocity after 2.52 s?

Homework Equations

F=m(Vf-Vi)/t

The Attempt at a Solution

This is how I used the equation

6240N=2500kg(Vf-25m/s)/(2.52s)

Solving for Vf I got 31.29m/s

The problem is that would mean the car's velocity is increasing, which it should be decreasing I think...

 

[temaire]

I think its because you forgot to make your acceleration negative.

 

[temaire]

I think it would have been better to use two formulas instead:

F = ma
a = (vf - vi)/t

 

[KatieLynn]

Yay thank you, I got the right answer once I did that :)

 

[KatieLynn]

Ooo one more question

to find the distance the car traveled in that problem I'm using the equation

d=1/2(Vf+Vi)t

d=1/2(-18.71m/s+25m/s)(2.52s)

Do I make the initial velocity of 25 m/s negative there? and the 18.71 should still be negative correct?

 

[KatieLynn]

Actually I realize they should both be negative velocities.

So last part of the question asks how long would it take for the car to come to a complete stop? Does that basically mean when does final velocity equal zero?

 

[temaire]

Yes it does.

 

[Shooting Star]

Actually I realize they should both be negative velocities.

Why are the velos negative? You should put accn as -ve. If you make the velos -ve, then accn should be +ve.