# Calculating distance, acceleration and time

• agargento
In summary, the conversation discusses a problem involving a truck traveling at a constant speed approaching a car stopped at a red light. The car then accelerates at a constant rate once the light turns green. The goal is to determine how close the truck comes to the car assuming it does not slow down. After some discussion and confusion about the numerical values, it is determined that the numbers are correct and the correct result is eventually found.
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?

Last edited by a moderator:
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.

 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)

BvU said:
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.

 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?

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.

BvU said:
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.

TomHart said:
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
agargento said:
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. :)

## What is the formula for calculating distance?

The formula for calculating distance is distance = speed x time. This means that the distance an object travels is equal to the speed at which it is traveling multiplied by the time it has been traveling for.

## How do you calculate acceleration?

Acceleration is calculated by dividing the change in velocity by the change in time. The formula for acceleration is a = (vf - vi) / t, where a is acceleration, vf is final velocity, vi is initial velocity, and t is time.

## What units are used to measure distance and time?

Distance is typically measured in meters (m) or kilometers (km), while time is measured in seconds (s) or hours (h). However, other units such as miles and minutes can also be used depending on the context.

## How does increasing acceleration affect an object's velocity?

Increasing acceleration will result in a greater change in velocity over a given period of time. This means that the object will be traveling faster than it was before.

## Can acceleration have a negative value?

Yes, acceleration can have a negative value. A negative acceleration, also known as deceleration, indicates that an object is slowing down. This can occur when the object is moving in the opposite direction of its initial velocity, or when it is experiencing a force that is causing it to slow down.

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