car stop speed

Question

The stopping distance d of a car after the brakes are applied varies directly as the square of the speed r. If a car traveling50
mph can stop in 110
?ft, how many feet will it take the same car to stop when it is traveling 100
?mph?

Answer 1

The speed doubles (50 ⇒ 100), therefore the distance shall be 4 times (2²) the distance, or 440 feet.

 

Or doing algebra:

 

d = r²k

110 = (50)²k

110 = 2500k

0.044 = k

 

d = (100²)(0.044)

d = 10000(0.044)

d = 440