Why is acceleration positive in this exercise?

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Homework Help Overview

The discussion revolves around a physics problem involving the calculation of acceleration and force for a car coming to a stop. The scenario includes a 1400kg car stopping from a speed of 35km/h and the force experienced by a 68kg occupant.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to calculate acceleration using the kinematic equation and expresses confusion over the sign of the acceleration and force. Some participants suggest that the problem may be asking for the magnitude of the force, which would be positive.

Discussion Status

The discussion is exploring the interpretation of the problem, particularly regarding the sign of the acceleration and force. Participants are questioning the assumptions made in the original poster's calculations and considering the possibility that the book's answer refers to the magnitude rather than the directional value.

Contextual Notes

There is uncertainty regarding the specific wording of the book's question and whether it explicitly asks for the magnitude of the force, which could clarify the confusion about the positive and negative values.

Frankenstein19
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Homework Statement


Calculate the acceleration of a 1400kg car if it can stop from 35km/h (9.72m/s) on a dime (diameter= 1.7cm) (0.017m). What is the force felt by the 68kg occupant of the car?

Homework Equations


v^2=v0^2+2a(x-x0)
F=ma

The Attempt at a Solution


ok after solving for a in that equation i get -2779m/s^2. when i go to calculate the force felt by the 68kg constituant, i'd just plug everything in, which would mean i'd get a negative force. the back of my book says that the answer is correct but instead of it being negative like i have it it is positive. WHY? why was the acceleration changed?
 
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It's difficult to tell without having the book, but it's likely that it was simply looking for the magnitude of the force, which would be positive.
 
axmls said:
It's difficult to tell without having the book, but it's likely that it was simply looking for the magnitude of the force, which would be positive.
Capture.PNG
the books question
Capture2.PNG


the answer
 
Yes, it looks like they're just looking for the magnitude of the force.
 

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