Instantaneous Velocity Car Problem

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Homework Help Overview

The problem involves a car's motion described by the equation x(t) = bt² - ct³, where the parameters b and c are given. The original poster seeks to calculate the average velocity over a specified time interval and later questions how to determine when the car comes to rest again.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss calculating average velocity and the need to derive the velocity function from the displacement equation. There is also a focus on determining the time when the car is at rest again.

Discussion Status

Some participants have provided guidance on calculating average velocity and deriving the velocity function. The original poster has expressed confusion regarding instantaneous velocity and the subsequent question about the car's rest state, indicating an ongoing exploration of the problem.

Contextual Notes

The discussion includes a transition from average velocity to instantaneous velocity, highlighting the need for clarity on definitions and calculations. There is also mention of using the quadratic formula to solve for time, suggesting a mathematical approach to the problem.

mopar969
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A car is stopped at a traffic light. It then travels along a straight road so that its distance from the light is given by x(t)= bt^2 -ct^3, where b = 2.40m/s^2 and c = 0.120 m/s^3. How do you calculate the average velocity of the car for the time interval t=0 to t=10.

I think that maybe I need to take the derivative of the equation but then what please help!
 
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Hello mopar969,
mopar969 said:
How do you calculate the average velocity of the car for the time interval t=0 to t=10.
If you only need to find the average velocity, simply calculate the total distance traveled, divided by the time it takes to get there.
 
Thank you. I can't believe that it was that simple. I was thinking instantaneous velocity that is why I was thinking about taking its derivative.
However, the question has another part to it that I am stuck on it says:

How long after stating from rest is the car again at rest? Please Help and Thanks.
 
You're searching the time t when the velocity is 0.

x(t)= bt^2 -ct^3, this finds the displacement as a function of time. You'll need another equation (related to this one) to find the velocity as a function of time.
 
Would I need the derivative of the displacement function?
 
Yes that would give you the velocity.
 
My next problem is how do I get t by itself?
 
Never mind I realized that I could use the quadratic formula.
 

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