The stopping distance d of a car after the

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 traveling 40 mph can stop in 70 ft, how many feet will it take the same car to stop when its traveling 80 mph

Answer 1

d = kr²

 

Get k from the given information.

 

k = d/r² = (70 ft) / (40 mph)² = 0.04375 ft/mph²

 

When r = 80 mph,

 

d = kr² = (0.04375 ft/mph²)(80 mph)² = 280 ft

Answer 2

Because the distance D varies directly with the square of the speed R, we can come up with the equation D=kR² where k is some constant.  We can solve for the constant k by plugging in the known values they gave us.  So we can come up with 70 = k(40²)  Solving for k, we come up with k=.04375.  Now that we know what k is, we can use it to solve for the distance of the last part by using D=(.04375)(80²).  Solving this we come up with D=280 feet.