Question

The police measured the skid marks made by a car that crashed into a tree. The formula used to approximate the distance in feet that it takes to stop a car after the brakes are applied for a car traveling at a rate of r miles per hour is d= 0.045 r^2 + 1.1 r . If the measurement gave a braking distance of 250 ft, was the driver exceeding the legal speed limit of 55 mi/h? Find the speed of the car before the brakes were applied.

Answer 1

Replace r in the equation with 55 mph and solve for d.

D = 0.045(55)^r + 1.1(55)

Simplify:

D = 0.045(3025) + 60.5

D = 136.125 + 60.5

D = 196.625

At the speed limit of 55 mph the skid mark would be 196.625 feet long.

Because the skid mark was greater than that it was going faster than 55 mph.

To find the speed the car was going replace d with 250 and solve for r:

250 = 0.045r^2 + 1.1r

Multiply both sides by 1000 to remove decimals:

250000 + 45r^2 + 1100r

Subtract 250000 from both sides

45r^2 + 1100 -250000 = 0

Use the quadratic formula to solve:

R = -1100 + sqrt(1100^2 14x45(-250000) /(2 x45)

R = 63.308 miles per hour

The speed of the car was 63.3 miles per hour