Fastest route through the forest

A problem was posted by the Danish newspaper, Ingeniøren, and it goes like this:

You are in the middle of a dense forest located at

$A$ . You need to get to

$C$ in the fastest way possible, and you can only change direction once. You can walk directly via

$AB$ to the dedicated walking path

$BC$ where you can walk fast, you can take the direct path through the forest (

$AC$ ) where you have to walk slower, or cross through the forest to the dedicated walking path (

$AD$ and then

$DC$ ).

Information given

Distances
- $|AB| =$ 300 m
- $|AC| =$ 1,000 m
Velocities
- $v_{AB} = 2$ m/s
- $v_{AD} = 2$ m/s
- $v_{AC} = 2$ m/s
- $v_{BC} = 5$ m/s

Length of line segments

We parameterise with $k = |BD|$ , the distance between $B$ and $D$ . That is, how much to walk on fast walking path before crossing into the forest.

Formulating using caracas:

AB <- as_sym('300')
AB
#> [caracas]: 300
AC <- as_sym('1000')
AC
#> [caracas]: 1000
BC <- sqrt(AC^2 - AB^2)
BC
#> [caracas]: 100⋅√91
k <- symbol('k') # |BD|
DC <- BC - k
AD <- sqrt(AB^2 + k^2)
AD
#> [caracas]:    ____________
#>              ╱  2         
#>            ╲╱  k  + 90000

So for a distance of $|AD|$ , you travel by 5 m/s, and then for a distance of $- k + 100 \sqrt{91}$ you travel by 2 m/s. Thus it takes $\frac{\sqrt{k^{2} + 90000}}{2}$ to travel $AD$ and $- \frac{k}{5} + 20 \sqrt{91}$ to travel $DC$ .

The question is: What is the fastest way to get from $A$ to $C$ ?

First, the total duration of the route is:

l <- AD/2 + DC/5
l
#> [caracas]:          ____________         
#>                    ╱  2                  
#>              k   ╲╱  k  + 90000          
#>            - ─ + ─────────────── + 20⋅√91
#>              5          2

lfun <- as_expr(l)
lfun
#> expression(-k/5 + sqrt(k^2 + 90000)/2 + 20 * sqrt(91))
ks <- seq(0, as_expr(AC), length.out = 100)
ls <- eval(lfun, list(k = ks))
plot(ks, ls, type = "l", xlab = "k", ylab = "Time A to C")

It looks like a minimum around $k = 150$ .

We find the analytical solution by first finding critical points:

dl <- der(l, k)
dl
#> [caracas]:         k           1
#>            ───────────────── - ─
#>                 ____________   5
#>                ╱  2             
#>            2⋅╲╱  k  + 90000
crit_points <- solve_sys(dl, k)
crit_points
#> Solution 1:
#>   k =  200⋅√21
#>       ───────
#>          7
best_k <- crit_points[[1]]$k
best_k
#> [caracas]: 200⋅√21
#>            ───────
#>               7

The type of the critical point is found by considering the Hessian:

eval(as_expr(der(dl, k)), list(k = as_expr(best_k)))
#> [1] 0.001283121

Thus the critical point is indeed a minimum as suggested by the plot.

The fastest route is thus obtained for $k = \frac{200 \sqrt{21}}{7} \approx 130.93 .$ It has a length of (in meters)

DC_best <- BC - best_k
AD_best <- sqrt(AB^2 + best_k^2)
AD_best
#> [caracas]: 500⋅√21
#>            ───────
#>               7
best_route <- AD_best + DC_best
best_route
#> [caracas]: 300⋅√21          
#>            ─────── + 100⋅√91
#>               7
as_expr(best_route)
#> [1] 1150.335

$\frac{300 \sqrt{21}}{7} + 100 \sqrt{91} \approx 1150.34$ and takes (in seconds)

best_l <- subs(l, "k", best_k)
best_l
#> [caracas]: 30⋅√21 + 20⋅√91
as_expr(best_l)
#> [1] 328.2651

$30 \sqrt{21} + 20 \sqrt{91} \approx 328.27$

The best route can be illustrated, too: