Question

A​ hot-air balloon, headed due east at an average speed of 30 miles per hour at a constant altitude of 95 ​feet, passes over an intersection​ (see the​ figure). Find an expression for its distance d​ (measured in​ feet) from the intersection t seconds later.

Answer 1

Step-by-step explanation:

1 mile = 5280 ft

1 hour = 3600 seconds (60 minutes of 60 seconds each).

the open question here is : do we need the ground distance, or really the line of sight distance (e.g. directly above the intersection the distance is 0 ft ground distance but 95ft line of sight distance) ?

based on the above conversions the speed

30 miles per hour = 30×5280 ft per 3600 seconds =

= 158,400 ft / 3600 s = 44 ft/s

so, the balloon moves away from the intersection 44 ft every second.

so, just for ground distance in ft we would have

d = 44t

for the line of sight distance we need to use Pythagoras, as we need to calculate the length of the Hypotenuse (= the line of sight) of the right-angled triangle of ground distance, height and line of sight.

the ground distance is again the same function as before (44t). height is constant (95 ft).

so,

d² = (44t)² + 95² = 1936t² + 9025

d = sqrt(1936t² + 9025)

so, e.g. after 1 second the line of sight distance would be

d = sqrt(1936×1 + 9025) = sqrt(1936 + 9025) =

= sqrt(10961) = 104.6947945... ft

that means although the ground distance increased by 44 ft, the line of sight distance increased only by roughly 10 ft.

that's how right-angled triangles work.

after 2 seconds d = 129.4951737... ft, so, roughly a 25 ft increase compared to 1 second. the change rate of the line of sight distance (the Hypotenuse) will get closer and closer to the (constant) change rate of the ground distance (but will never fully reach it - only in infinity ...).