Calculating distance, acceleration and time

[agargento]

Homework Statement

To save fuel, some truck drivers try to maintain a constant speed when possible. A truck traveling at 28.0 km/hr approaches a car stopped at the red light. When the truck is 110.7 meters from the car the light turns green and the car immediately begins to accelerate at 3.8 m/s2. How close does the truck come to the car assuming the truck does not slow down?

Homework Equations

Equations used:

V=V₀+at
and r(t)=r₀+v₀t+0.5a²

The Attempt at a Solution

my attempt: http://imgur.com/RHuv2tq

I got an incorrect answer, what did i get wrong?

 

[BvU]

Did I manage to get my reply across before the good spirit that watches over us took action ?

I asked you to check Dcar: that's not the expression for uniformly accelerated linear motion.

[edit] Oh, I see: you mixed them up. Nevertheless, answer's incorrect: you want to check what you do with the signs

By the way, the numerical values in this exercise are strange (truck speed very low wrt distance)

 

[agargento]

Did I manage to get my reply across before the good spirit that watches over us took action ?

I asked you to check Dcar: that's not the expression for uniformly accelerated linear motion.

[edit] Oh, I see: you mixed them up. Nevertheless, answer's incorrect: you want to check what you do with the signs

By the way, the numerical values in this exercise are strange (truck speed very low wrt distance)

Sorry, I didn't completely understand where the problem is?

 

[TomHart]

It looks like you have the car and truck equations reversed. Also, I think your final equation would indicate that the vehicles are headed toward each other.

 

[TomHart]

By the way, the numerical values in this exercise are strange (truck speed very low wrt distance)

You know, that's what I thought when I worked out the result - that the numbers were kind of strange. I was wondering if the acceleration number might have been wrong; it seems kind of high. But you're right that the truck's speed seems very low also. @agargento, please check that you copied all of the values correctly.

 

[agargento]

You know, that's what I thought when I worked out the result - that the numbers were kind of strange. I was wondering if the acceleration number might have been wrong; it seems kind of high. But you're right that the truck's speed seems very low also. @agargento, please check that you copied all of the values correctly.

Hey, I got the correct result eventually, the numbers are correct. I've built an equation of their distance x(t) = 110.7 + 0.5 * 3.8 * t^2 -(28/3.6) * t and used the time I found to find the minimal distance, and it was correct.

 

[TomHart]

Hey, I got the correct result eventually, the numbers are correct.

Okay good. Glad to hear that. My personal opinion though is that the problem writers should add a little more drama and suspense. :)