Acceleration Problem: Solving for Constant Acceleration with Kinematic Equations

[nordqvist11]

Homework Statement

I'd just like someone to see if I answered correctly. I'm having a bit of difficulty grasping the vagueness of the displacement in these questions I've been getting in my textbook. From what I gather the displacement is relative to the vector points, (the two x points in kinematic equations). However, how am I to know what direction the car is traveling relative to the x plane if it doesn't specify the point in a question? I don't know if you guys get me.. The acceleration I found in the question below, could be positive or negative, it doesn't specify enough. Or am I wrong?

A car slows down from 23 m/s to rest in a distance of 85 m. What was its acceleration, assumed constant?

Homework Equations

The kinematic equations for constant acceleration.

I attempted to solve it with the following:

finalV^2=initialV^2+2a(finalX-initialX)

The Attempt at a Solution

Here is an image of my attempted work. I got -3.1 m/s^2 as my answer.

 

[lewando]

Good work by you!

 

[nordqvist11]

Yes! Thanks

 

[lewando]

The acceleration I found in the question below, could be positive or negative, it doesn't specify enough. Or am I wrong?

A car slows down from 23 m/s to rest in a distance of 85 m. What was its acceleration, assumed constant?

Sorry, I did not read your question carefully enough.

V_0x = +23m/s (assumedly in the +x direction)
x₀ = 0
x = 85

Since the car is decelerating, the acceleration vector will be in the opposite direction of the velocity vector, hence the appropriateness of the "-" in front of 3.1m/s²

 

[nordqvist11]

Very helpful, thank you.