Estimating Car Speed from Skid Marks

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    Car Marks Speed
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Homework Help Overview

The problem involves estimating the speed of a car just before it begins to brake, based on the length of skid marks left on the highway and the assumed deceleration. The subject area pertains to kinematics and the application of equations of motion under constant acceleration.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster questions their approach to the problem, particularly regarding the distinction between speed and velocity. Some participants discuss the relationship between speed and velocity, while others suggest reviewing the signs used in the kinematic equation.

Discussion Status

The discussion includes various interpretations of the terms speed and velocity, with some participants providing insights into the kinematic equation. There is an ongoing exploration of the correct application of the equation and the implications of sign conventions.

Contextual Notes

Participants are navigating the nuances of the problem, including the definitions of speed versus velocity and the correct usage of variables in the kinematic equation. There is an acknowledgment of potential confusion regarding the signs in the calculations.

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Homework Statement


In coming to a stop a car leaves skid marks 92m long on the highway. Assuming a decceleration of 7.00 m/s^2 , estimate the SPEED of the car just before braking.

I have a question though, is my approach ok? You can see my confusion in various parts of my procedures below, I guess it's the fact that it says find speed not velocity.

Homework Equations


Kinematic Equation for constant acceleration.



The Attempt at a Solution



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Speed=velocity lol
 
I got -35m/s as my velocity though
 
Speed is the magnitude of velocity and has no direction.

EDIT: Assuming you are using the formula: v^2=u^2+2as you may need to check what you are using as v and u and be careful of your signs.

0 = u^2 +2as
u^2=-2as

Does that make sense?
 
Last edited:

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