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avg1
(55 km/h)( /2) (90 km/h)( / 2)
72.5 km/h
DT
T
s
TT
+
==
=
which should be rounded to 73 km/h.
(b) Using the fact that time = distance/speed while the speed is constant, we find
avg2
/2
55 km/h
90 km/h
68.3 km/h
DD
s
T
=
+
which should be rounded to 68 km/h.
(c) The total distance traveled (2
D
) must not be confused with the net displacement (zero).
We obtain for the twoway trip
avg
72.5 km/h
68.3 km/h
2
70 km/h.
D
s
+
(d) Since the net displacement vanishes, the average velocity for the trip in its entirety is
zero.
(e) In asking for a
sketch
, the problem is allowing the student to arbitrarily set the
distance
D
(the intent is
not
to make the student go to an Atlas to look it up); the student
can just as easily arbitrarily set
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 Spring '06
 Stoler
 Physics

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