Sports car Acceleration problem

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Homework Help Overview

The discussion revolves around a physics problem involving the acceleration of a sports car that decelerates to a stop. The original poster presents a scenario where the car travels a certain distance at a constant speed before braking, and they seek to calculate the acceleration in terms of g's.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the calculation of acceleration, questioning the use of signs and the interpretation of the problem's parameters. There are attempts to verify the correctness of the original poster's calculations and the potential impact of rounding and significant figures.

Discussion Status

Some participants offer insights into possible errors in the original poster's approach, particularly regarding the use of negative signs and the interpretation of the problem's direction. Others express frustration with the web-based homework system and its limitations, while some explore related problems, indicating a broader engagement with the topic.

Contextual Notes

There is mention of constraints related to the web-based homework system, including limited attempts to submit answers and the potential for errors due to rounding and significant figures. Additionally, there is a question about the value of g used in the calculations.

JDR 007
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Homework Statement



A sports car moving at constant speed travels 120 m in 4.7 s. If it then brakes and comes to a stop in 4.0 s, what is its acceleration? Express the answer in terms of g's, where g = 9.80 m/s2. (Take the positive direction to be the direction of travel.)

Homework Equations



Change in speed/time of change

The Attempt at a Solution


120/4.7=25.53m/s
end speed 0m/s-start speed 25.53m/s divided by time 4s= -6.38m/s^2 divided by 9.8m/s2= -.651 g

I got this wrong...what did I do wrong?
 
Last edited:
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I think its probably because you forgot the minus sign. The question specifically states which direction is positive.
 
Looks right to me.
 
I used a minus sign...still wrong. It is a web based homework system and it is horrible. we have 4 trys to get it right...so i used a - sign then a + sign then scientific notation then changed my sig figs...so it says I am wrong but I don't see how.
 
JDR 007 said:
I used a minus sign...still wrong. It is a web based homework system and it is horrible. we have 4 trys to get it right...so i used a - sign then a + sign then scientific notation then changed my sig figs...so it says I am wrong but I don't see how.

Double check that you posted the question exactly as it appears... you might have mixed up the numbers or something...
 
I copy and pasted the question it is exact
 
So the question gave g as 9.80m/s2? I'd have expected 9.81m/s2 if it was to two decimal places.
 
I emailed my teacher And I will see what she says. I really hate these web assignments because they aren't multiple choice (hence wrong sigs, diff. calculators etc... will mess answers up due to rounding)
 
A car slows down from 23 m/s to rest in a distance of 91 m. What was its acceleration, assumed constant?

I only have one more try on this one... is -2.91 right?
 
  • #10
JDR 007 said:
A car slows down from 23 m/s to rest in a distance of 91 m. What was its acceleration, assumed constant?

I only have one more try on this one... is -2.91 right?

Yes, that's what I get rounded to 2 decimal places.
 

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