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Question: 1 an automobile travels on a straight road for a...

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1. An automobile travels on a straight road for a distance d1 = 20 km at a speed s1 =
+30 km/h. Call this motion in the +x direction. It then reverses direction and travels
a distance d2 = 50 km at a speed s2 = +60 km/h. (Note: The quantities s1 and s2 are
both positive because they both represent the speed of the car. “Speed” in physics is an
inherently positive quantity as it is the magnitude of the velocity. “Velocity” in physics
is a quantity that contains both speed and direction information. For one-dimensional
motion direction can be indicated with a “+” or “−” sign. In this problem we would
say the velocity of the car is initially v1 = +30 km/h, and is v2 = −60 km/h after
changing direction.)
Givens: d1 = 20 km, s1 = 30 km/h, d2 = 50 km, s2 = 60 km/h.
(a) What is the average velocity of the car during the full 70 km trip? Work sym-
bolically to derive vavg(d1, d2, s1, s2) then calculate a numerical result. (b) What is
the average speed? Work symbolically to derive savg(d1, d2, s1, s2) then calculate a
numerical result.
Ans: (a) −20 km/h, (b) 47 km

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