Chapter 4.7, Problem 52IP

To determine

To explain the car speeding before applying the brake in a dry, concrete road under the given condition.

Answer to Problem 52IP

The car will skid on the dry, concrete surface.

Explanation of Solution

Given information: $s=\sqrt{30fd}$ , where

  $\begin{array}{l}s-\text{speed}\text{of}\text{the}\text{car in}\text{miles/hr}\\ d-\text{distance of the car skidded in feet}\\ f-\text{ friction }\end{array}$

Here,

  $s=55\text{miles/hr;}d=100\text{ feet}$

  $\begin{array}{l}55=\sqrt{30\times f\times 100}\\ 55=10\sqrt{30\times f}\\ \frac{11}{2}=\sqrt{30\times f}\\ \frac{121}{4}=30\times f\\ f=\frac{121}{120}=1.008\end{array}$

Hence, the value of the friction is greater than permissible limit. The car will skid.