How Does Kinetic Friction Affect Skid Mark Length and Initial Car Speed?

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Homework Help Overview

The discussion revolves around a physics problem involving kinetic friction, skid marks, and the initial speed of a car involved in an accident. The scenario includes a police officer's estimation of the car's speed and the length of skid marks left on the road.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss calculating the deceleration of the skidding car and relate it to the length of the skid marks. There are attempts to understand the relationship between kinetic energy change and the skid mark length, as well as the force of friction involved.

Discussion Status

Some participants are attempting calculations based on the provided information, while others are questioning the methods and reasoning behind their attempts. Guidance has been offered regarding applying Newton's second law to derive acceleration and initial velocity.

Contextual Notes

There are indications of confusion regarding the calculations, particularly in determining time and relating it to the skid mark length. The problem context includes specific values for speed and friction, but there may be missing information necessary for complete resolution.

Robertoalva
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1. A police officer investigating an accident estimates from the damage done that a moving car hit a stationary car at 25 km/h. If the moving car left skid marks 44 m long, and if the coefficient of kinetic friction is 0.68, what was the initial speed of the moving car?



Homework Equations


fk = μk N


F = ma

The Attempt at a Solution


drew a free body diagram, converted 25 km/h to 6.94 m/s
 
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Can you calculate the deceleration of the skidding car?
 
i tried and i do not know how. i tried getting the time by multiplying some number bye 44 in order to give me 6.94 but I didn't get the exact number... it is approaching to 6.3something.
 
Apply Newton 2 to your free-body diagram to get the acceleration of the car.
Then you can calculate the initial velocity.
 
Robertoalva said:
1. A police officer investigating an accident estimates from the damage done that a moving car hit a stationary car at 25 km/h. If the moving car left skid marks 44 m long, and if the coefficient of kinetic friction is 0.68, what was the initial speed of the moving car?



Homework Equations


fk = μk N


F = ma

The Attempt at a Solution


drew a free body diagram, converted 25 km/h to 6.94 m/s
How is the change in kinetic energy of the car (ie. from just before the skid starts until it strikes the car) related to the length of the skid mark and the force of friction?

AM
 

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