The skid marks from a car involved in an accident measured 85ft. To the nearest tenth, what was the speed of the car before the brakes were applied?

Question

Miles per hour

Answer 1

There is not enough information to answer the question. We need to also know the braking acceleration or the braking force and the vehicle mass, from which we can get the braking acceleration. Another way to solve it would require knowing the time it took to decelerate to a full stop. Suppose you know the braking acceleration is a, where a is a negative number. From kinematics,

v_f² - v_i² = 2ad

v_f = final speed = 0 (full stop)
v_i = initial speed = ?
d = stopping distance (given)

v_i = √(-2ad)

Note that since a is negative, the argument of the square root is positive. If you do not know a but you know the braking force F and vehicle mass m, you can get a from Newton's 2nd law:

a = F/m

Anyway, we need more information to answer the question.