The average a-velocity depends on only the total displacement Ax x2-x Fas A ba t

Question

The average a-velocity depends on only the total displacement Ax x2-x Fas A ba t occurs during the time interval , not on what happens during a motorcycle might have raced past the dragster at Pre the time interval. At time Fa int Pi in Fig. 2.1, then slowed down to pass through point P2 at the same time t2 as the dragster. Both vehicles have the same displacement during the same time interval and so have the same average x-velocity sured in meters per second, or m/s (Table 2.2). Other common unit knots (I knot -1 nautical mile/h 6080 ft/h). If distance is given in meters and time in seconds, average velocity is mea- are kilometers per hour (km/h), feet per second (ft/s), miles per hour (mi/h), and TEST YOUR UNDERSTANDING OF SECTION 2.1 Each of the following five trips takes one hour. The positive x-direction is to the east. (i) Automobile A travels 50 km due east. (ii) Automobile B travels 50 km due west. (iii) Automobile C travels 60 km due east, then turns around and travels 10 km due west. (iv) Automobile D travels 70 km due east. (v) Automobile E travels 20 km due west, then turns around and travels 20 km due east. (a) Rank the five trips in order of average x-velocity from most positive to most negative. (b) Which trips, if any, have the same average x-velocity? (c) For which trip, if any, is the average x-velocity equal to zero? 2.2 INSTANTANEOUS VELOCITY Sometimes average velocity is all you need to know about a particle's moti For example, a race along a straight line is really a competition to see whose erage velocity, Vavx, has the greatest magnitude: The prize goes to the compe who can travel the displacement from the start to the finish line in the s est time interval, At (Fig. 2.4) But the average velocity of a particle during a time interval can't tell us fast, or in what direction, the particle was moving at any given time duri nterval. For that we need to know the instantaneous velocity, or the velo specific instant of time or specific point along the path. How long is an instanf? You might use the phrase "It lasted just an refer to something that spanned a very short time interval. But in physics an in o duration at all; it refers to a single value of time. To find the instantaneous velocity of the dragster in Fig.2

Explanation / Answer

2. 1 t = 1hr

a. 50 km east

b. 50 km west

c. 60 km east + 10 km west

d. 70 km east

e. 20 km west + 20 km east

a. average x velocity = x displacement/time

va = 50 km/h

vb = -50 km/h

vc = 50 km/h

vd = 70 km/h

ve = 0 km/h

hence

vd > va = vc > ve > vb

b. trip a and c have same average x velocity

c. for trip e the average velocity is 0

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