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

[Robertoalva]

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

 

[haruspex]

Can you calculate the deceleration of the skidding car?

 

[Robertoalva]

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.

 

[ap123]

Apply Newton 2 to your free-body diagram to get the acceleration of the car.
Then you can calculate the initial velocity.

 

[Andrew Mason]

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