Basic question about velocity

Discussion in 'Physics' started by krow, Jul 6, 2011.

  1. krow

    Thread Starter Member

    May 25, 2010
    Hey guys,

    I think my brain is malfunctioning after studying too hard for my exams, I'm embarrassed to say I'm studying engineering and I don't know how to solve this problem.

    In an car accident, the police find that the car applied the brakes at time t which makes it stop 40 meters after, with that data, how can we calculate the velocity at which the car was travelling? am I missing data?
  2. nuckollsr


    Dec 17, 2009
    Yes, you need more data. Coefficient of friction for tires against the road determine the retarding force for the sliding vehicle. A sample solution might take the form Distance = 1/2(acceleration) x t(squared). Assume coefficient of friction = .7 then acceleration is ~21 f/s/s. Distance traveled is given at 131 feet (40m). So time to stop is on the order of 3.5 seconds.

    Velocity at time of brake application is a work-backward from zero velocity where v = at or 21 x 3.5 which is on the order of 73.5 f/s. I used to sell road friction measurement equipment for the purpose of getting initial speeds of vehicles at time of brake application based on skid marks. Anti-lock brakes sorta shot that business in the between the eyes!

    The numbers I cited were "on the order of" . . . you should refine them to more nearly match your given data and probably convert to the mks values. I couldn't use mks in court if I wanted folks to understand the findings.
    Last edited: Jul 6, 2011