218 Chapter 8 Time and Time-Related Parameters
Example 8.3 A car starts from rest and accelerates to a speed of 100 km/h in 15s. The acceleration dur-
ing this period is constant. For the next 30 minutes, the car moves with a constant speed of
100 km/h. At this time the driver of the car applies the brake and the car decelerates to a full stop
in 10s. The variation of the speed of the car with time is shown in Figure 8.9. We are interested
in determining the total distance traveled by the car and its average speed over this distance.100
15 1815
Time (s)
1825
Speed (km/h)
■Figure 8.9 The variation of the speed of the car with time for Example 8.3.
During the initial 15 seconds, the speed of the car increases linearly from a value of zero
to 100 km/h. Therefore, the average speed of the car is 50 km/h during this period. The dis-
tance traveled during this period isDuring the next 30 minutes (1800 s), the car moves with a constant speed of 100 km/h and
the distance traveled during this period isBecause the car decelerates at a constant rate from a speed of 100 km/h to 0, the average speed
of the car during the last 10 seconds is also 50 km/h, and the distance traveled during this
period isThe total distance traveled by the car is
And finally, the average speed of the car for the entire duration of travel is
Vaverage
distance traveled
time
50347.2
1825
27.56 m/s99.2 km/h
dd 1 d 2 d 3 208.3 50000 138.950347.2 m50.3472 km
d 3 1 10 s2a 50
km
h
ba
1 h
3600 s
ba
1000 m
1 km
b138.9 m
d 2 1 1800 s2a 100
km
h
ba
1 h
3600 s
ba
1000 m
1 km
b50000 m
d 1 1 time 21 average speed 2 1 15 s2a 50
km
h
ba
1 h
3600 s
ba
1000 m
1 km
b208.3 m
Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀