# Thread: Problem of direct variation

1. ## Problem of direct variation

A car is traveling at speed s (in mph) on a dry asphalt road, and the brakes are suddenly applied. The stopping distance d (in feet) varies directly as the square of the speed s. If a car traveling at 60 mph can stop in 120 feet, what is the stopping distance of a car traveling at 70 mph?

I believe the correct answer is 140 feet. Is this right?

2. Originally Posted by mt_lapin
A car is traveling at speed s (in mph) on a dry asphalt road, and the brakes are suddenly applied. The stopping distance d (in feet) varies directly as the square of the speed s. If a car traveling at 60 mph can stop in 120 feet, what is the stopping distance of a car traveling at 70 mph?

I believe the correct answer is 140 feet. Is this right?
no. we have $d = ks^2$ for some constant k

we are told, when $d = 120,~s = 60$. use this fact to solve for k.

then plug in the value for k into the original equation. can you now find d if s = 70?