2 A hare and a tortoise compete in a race over a straight course
3 $1.00\U{km}$ long. The tortoise crawls at a speed of $0.200\U{m/s}$
4 toward the finish line. The hare runs at a speed of $8.00\U{m/s}$
5 toward the finish line for $0.800\U{km}$ and then stops to tease the
6 slow-moving tortoise as the tortoise eventually passes by. The hare
7 waits for a while after the tortoise passes by and then runs toward
8 the finish line again at $8.00\U{m/s}$. Both the hare and the tortise
9 cross the finish line at exactly the same instant. Assume both
10 animals, when moving, move steadily at their respective
11 speeds. \Part{a} How far is the tortoise from the finish line when
12 the hare resumes the race? \Part{b} For how long in time was the hare
17 Sometimes it is useful to draw a graph to get a feel for what's going
25 real v_scale = 10; // increase v_scale to decrease time spread
27 real L = 1e3*u; // length of race
28 real p = 0.8e3u; // hare pause location
29 real vt = 0.2 * v_scale; // tortoise velocity
30 real vh = 8 * v_scale; // hare velocity
31 real T = L / vt; // the tortoise walks the whole time
32 pair f = (L,T); // finish
33 pair h1 = (p, p/vh); // hare pause event
34 pair h2 = (p, T-(L-p)/vh); // hare restart event
39 Vector T1 = Vector((0,0), mag=length(f), dir=degrees(f), "tortoise");
40 T1.draw(rotateLabel=true, labelOffset=-f/2);
42 Vector H1 = Vector((0,0), mag=length(h1), dir=degrees(h1), "hare");
43 H1.draw(rotateLabel=true, labelOffset=-h1/2);
44 Vector H2 = Vector(h1, mag=length(h2-h1), dir=degrees(h2-h1), "pause");
45 H2.draw(rotateLabel=true, labelOffset=-(h2-h1)*2/3);
46 Vector H3 = Vector(h2, mag=length(f-h2), dir=degrees(f-h2), "hare");
47 H3.draw(rotateLabel=true, labelOffset=-(f-h2)/2);
55 The final distance run by the hare is
57 x_\text{h,2} = L - \Delta x_\text{h,1}
61 t_\text{h,2} = \frac{L - x_\text{h,1}}{v_\text{h}}
63 In this time, the tortoise covers
65 x_\text{t,2} &= v_\text{t} \cdot t_\text{h,2}
66 = v_\text{t}\frac{L - x_\text{h,1}}{v_\text{h}}
67 = \frac{v_\text{t}}{v_\text{h}}(L - x_\text{h,1}) \\
68 &= \frac{0.200\U{m/s}}{8.00\U{m/s}}(1.00\U{km} - 0.800\U{km})
73 The hare is running for
75 t_\text{h,run} = \frac{L}{v_\text{h}}
77 The tortoise is running for
79 t_\text{t,run} = \frac{L}{v_\text{t}}
81 and the tortoise is running for the whole race, so the hare pauses for
83 t_\text{h,pause} &= t_\text{t,run} - t_\text{h,run}
84 = \frac{L}{v_\text{t}} - \frac{L}{v_\text{h}}
85 = \frac{1.00\U{km}}{0.200\U{m/s}} - \frac{1.00\U{km}}{8.00\U{m/s}} \\
86 &= \ans{4.88\U{ks}} = 1.35\U{hours}