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CHAPTER
2
Motion in One Dimension
1* ·
What is the approximate average velocity of the race cars during the Indianapolis 500?
Since the cars go around a closed circuit and return nearly to the starting point, the displacement is nearly zero,
and the average velocity is zero.
2
·
Does the following statement make sense? “The average velocity of the car at 9 a.m. was 60 km/h.”
No, it does not. Average velocity must refer to a finite time interval.
3
·
Is it possible for the average velocity of an object to be zero during some interval even though its average
velocity for the first half of the interval is not zero? Explain.
Yes, it is. In a round trip,
A
to
B
and back to
A
, the average velocity is zero; the average velocity between
A
and
B
is not zero.
4
·
The diagram in Figure 221 tracks the path of an object moving in a straight line. At which point is the
object farthest from its starting point?
(
a
)
A
(
b
)
B
(
c
)
C
(
d
)
D
(
e
)
E
(
b
) Starting point is at
x
= 0; point
B
is farthest from
x
= 0.
5* ·
(
a
) An electron in a television tube travels the 16cm distance from the grid to the screen at an average
speed of 4
×
10
7
m/s. How long does the trip take?
(
b
) An electron in a currentcarrying wire travels at an
average speed of 4
×
10
5
m/s. How long does it take to travel 16 cm?
(
a
) From Equ. 23,
∆
t
=
∆
s
/(av. speed)
(
b
) Repeat as in (
a
)
∆
t
= (0.16 m)/(4
×
10
7
m/s) = 4
×
10
9
s = 4 ns
∆
t
= (0.16 m)/(4
×
10
5
m/s) = 4
×
10
3
s = 4 ks
6
·
A runner runs 2.5 km in 9 min and then takes 30 min to walk back to the starting point.
(
a
) What is the
runner’s average velocity for the first 9 min?
(
b
) What is the average velocity for the time spent walking?
(
c
)
What is the average velocity for the whole trip?
(
d
) What is the average speed for the whole trip?
Take the direction of running as the positive direction.
(
a
) Use Equ. 22
(
b
) Use Equ. 22
(
c
)
∆
x
= 0
(
d
) Total distance = 5.0 km;
∆
t
= 39 min
v
av
= (2.5 km)/[(9 min)(1 h/60 min)] = 16.7 km/h
v
av
= (2.5 km)/(0.5 h) = 5.0 km/h
v
av
= 0
Av. speed = (5.0 km)/[(39 min)(1 h/60 min)]
= 7.7 km/h
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·
A car travels in a straight line with an average velocity of 80 km/h for 2.5 h and then with an average
velocity of 40 km/h for 1.5 h.
(
a
) What is the total displacement for the 4h trip?
(
b
) What is the average
velocity for the total trip?
(
a
) 1. Find the displacements of each segment;
Use Equ. 22
2. Add the two displacements
(
b
) Use Equ. 22
S
1
= (80 km/h)(2.5 h) = 200 km;
S
2
= (40 km/h)(1.5 h) = 60 km
S
tot
= 260 km
v
av
= (260 km)/(4 h) = 65 km/h
8
·
One busy air route across the Atlantic Ocean is about 5500 km. (
a
) How long does it take for a supersonic
jet flying at 2.4 times the speed of sound to make the trip? Use 340 m/s for the speed of sound.
(
b
) How long
does it take a subsonic jet flying at 0.9 times the speed of sound to make the same trip?
(
c
) Allowing 2 h at
each end of the trip for ground travel, checkin, and baggage handling, what is your average speed door to door
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 Spring '09
 Eduardo
 Physics

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