Question

An automobile traveling at the rate of 30 ft/sec is approaching an intersection. When the automobile is 120 ft. from the intersection, a truck, traveling at the rate of 40 ft/sec on a road at right angles with the automobile’s road crosses the intersection. How long after the truck leaves the intersection the two vehicles are closest?

Answer 1

Let the time passed since the truck went through the intersection be t seconds
Make a diagram
Let the distance between them be D ft
I see that

D^2 = (40t)^2 + (120-30t)^2
= 1600t^2 + 14400 - 720t + 900t^2
= 2500t^2 - 7200t + 14400

2D dD/dt = 5000t -7200
but at a minimum of D , dD/dt = 0 , so

5000t = 7200
t = 7200/5000
= 1.44 sec after the truck entered the intersection

check:
when t = 0 , D = 120
when t = 1 , D = 98.5
when t = 1.44 , D = 96
when t = 1.5 , D = 96.05 , getting bigger again

Answer 2

thx reiny

Answer 3

thanks mike