Starting from rest, a dragster travels a straight 1/4 mi racetrack in 6.70 s with constant acceleration. what is its velocity when it crosses the finish line?

Explanation:

We have equation of motion s = ut + 0.5 at²

        Initial velocity, u = 0 m/s

        Acceleration, a = ?

        Time, t = 6.70 s      

        Displacement, s = 1/4 mi = 0.25 mi = 400 m

     Substituting

                      s = ut + 0.5 at²

                      400 = 0 x 6.70 + 0.5 x a x 6.70²

                      a = 17.82 m/s²

     Now we have equation of motion v = u + at

     Initial velocity, u = 0 m/s

     Final velocity, v = ?

     Time, t = 6.7 s

     Acceleration, a = 17.82 m/s²

     Substituting

                      v = u + at  

                      v = 0 + 17.82 x 6.7

                      v = 119.40 m/s

Its velocity when it crosses the finish line is 119.40 m/s

conservation of mass

mass before something happens = mass afterwards