Calculating Car Accident Speed: 100ft skid marks, 0.75g deceleration

[herbally]

Homework Statement

Investigating the scene of a car accident you measure the skid marks of one of the cars to be 100.0ft long. Assuming that the maximum deceleration on dry pavement is 0.75g, what was the minimum speed of the car(in mph) when it slammed on the brakes?

Homework Equations

x=x0 + v0t +1/2at^2
v=v0 + at

The Attempt at a Solution

I know I need to solve for t, but I can't seem to wrap my ahead around setting up the equations for these types of problems. A helpful nudge would be appreciated.

 

[Xisune]

You don't need to solve for time, you are asked to solve initial velocity. You are given 3 values, displacement, acceleration, and final velocity.

 

[herbally]

I was under the impression that most of these problems begin with solving for time. I'm really in the dark on this stuff. My instructor leaves much to be desired.

 

[Martinet]

Have you been taught all of the basic kinematic equations?

If so, look at each of them. You'll find that each one has a different variable missing.

Knowing this all you have to do is look for the equation with all the variables you're given and the variable you need and then use that.

 

[herbally]

This is what I did...let me know if I'm on the right track.

I used v_f² + v₀² = 2as.

so v₀² = 2(7.35m/s²)(30.48m)
v₀ = 21.2m/s

I think that's right, but I'm not sure. Then of course I have to convert to mph. Am I close?

 

[Xisune]

Yes, that is correct.