68. For each rate, we use distance d = vt and convert to SI using 0.0254 cm = 1 inch (from which we derive
the factors appearing in the computations below).
(a) The total distance d comes from summing
d
1
=
120
steps
min
0.762 m/step
60 s/min
(5 s) = 7.62 m
d
2
=
120
steps
min
0.381 m/step
60 s/min
(5 s) = 3.81 m
d
3
=
180
steps
min
0.914 m/step
60 s/min
(5 s) = 13.72 m
d
4
=
180
steps
min
0.457 m/step
60 s/min
(5 s) = 6.86 m
so that d = d
1
+ d
2
+ d
3
+ d
4
= 32 m.
(b) Average velocity is computed using Eq. 2-2: v
avg
= 32/20 = 1.6 m/s, where we have used the fact
that the total time is 20 s.
(c) The total time t comes from summing
t
1
=
8 m
120
steps
min
0.762 m/step
60 s/min
= 5.25 s
t
2
=
8 m
120
steps
min
0.381 m/step
60 s/min
= 10.5 s
t
3
=
8 m
180
steps
min
0.914 m/step
60 s/min
= 2.92 s
t
4
=
8 m
180
steps
min
0.457 m/step
60 s/min
= 5.83 s
so that t = t
1
+ t
2
+ t
3
+ t
4
= 24.5 s.
(d) Average velocity is computed using Eq. 2-2: v
avg
= 32/24.5 = 1.3 m/s, where we have used the
fact that the total distance is 4(8) = 32 m.