r
*
fhapter ? I Motion Along
a Straight Line
7 In Fig. 2-20, a cream tan_
gerine is thrown directlv
upwarrl past thrce cuenlv
sprced windows of
xy : -.20 m. The signs of the particle,s initial
velocity v11 (at
time tu) and constant acceleration a are. respectl;.1;,;;;
"qu.t
heights. Rank the windows
ac_
to (a) the average
o[ thc cream trngcrine
cording
specd
never pass through the origin?
I Hanging over the railing of
a bridge, you drop an egg (no
initial velocity) as you throw a
second egg downward. Which
curves in Fig.2-21 give the velocity u(l) for (a) rhe dropped
while passing them, (b) the
time the cream tangerine takes
Io pass them. (c) the mngni_
tude of the acceleration of the
c_ream tangerine while passing
them, and (d) the change Au in
the speed of the cream tanger_
ine during the passage, greatest
lirst.
S Al I along an
&
!#
55M
@-*!e
!s6
$es"
ol
e-4
0. a particle moving
axis is at position
r
cgg and (b) rhe lhrown
(Curves
SsS"
f
.#fi
and G.)
FlG.
?-21 Question
9.
Tutoring problem avaiiabre (at instructor's
discretion) in wireypLL)S andwebAssign
Worked-out solution available in Student Solutions
ManuaJ
WWW Worked-out solution is ar
Number of dots ind jcates levei of problem
difficulty
ILW lnteractive solution is a1
Additional information avairabre in The Ftying
Circus of physics and at fryingcircuso{physics.com
Average Velocity and Average Speed
An automobile travels on a straighl .nu.l fo.
40 km at
tie positive r di-
SSM WM,W
ofi A car travels up a hill a1 a constant spee<J
of 40 km/h and
returns down the hill at a constant speed
of 60 km/h. Calculate
the average speed for the round trip.
e$
During a harcl sneeze. your eyes might shut lor
0.50 s. If
you are driving a car at 90 km/h during such
a sneeze, how far
does the car move during that time?
a4
1992 world speed record for a bicycle
(human pow_
.The
ered vehicle) was set by Chris Huber. His
time through the
measured 200 m stretch was a sizzling 6.-509
s, at which he
commented, .,Cogito ergo zooml', (I think,
therefore I go
fastl). In 2001, Sam Whittingham beat Huber,s
record by
19.0 km/h. What was Whittingham,s time
through the 200 m?
The position of an object moving along
: 3t - 4t2 + /3, where x is in meters and ranin x axis is given
seconds. Find
n^os;ti9n of the object.ar rhe following values of r: (a) 1 s,
by -r
(b) 2 s, (c) 3 s, an<1 (d) 4 s. (e) Wf,at is the objecr,s
: 0 and t':' 4 s? (f) What is its aver_
age velocity for the time interval from'l :2
s to t : 4 s,?
(g) Graph x versus / for 0 <t<4s and indicate
how the
displacement between /
answer for (t) can be found on the
*$
egg..,
and
Questjon 7.
rection.) (b) What is the average speed? (c)
Graph -t vcrsus I
and indicate how the average uetocity is
founcl on the graph.
jf3
A
0
B are parallel;
so are C, D, and E; so are F
30 km/h.It then continues in the same
dlrecticn tor another 40
km/h. (a) What is the average velocitf of
J<m 1t.60
rhe car clur_
ing this 80 km trip? (Assume that it moves
in
e$
i;;;
+, +; (2) +, -; (3) _. +;
ti; :, _ rn which
situations will the particle (a)
r_top momentarily, (b) pass
through rhe origin, and (.)
situations: (1)
graph.
ssM
Compute your average velocity in the followr.ng
two
cases: (a) You walk 73.2 m at a speed of
1.22 mls and then run
73.2m at a speed of 3.0-5 m/s along a straight
track. (b)
You walk for l.00 min at a speed of 1.22
mls ancl the n run for
min at 3.05 m/s along a straight track. (c)
Graph.r versus /
for both cases and inclicate howlh. ou.rug"
u.lo.itr. is found
on the graph.
1.00
q.*7
In 1 km racesr runner 1 on track 1 (with time
2 min.27.9-5
s) appears to be faster than runner
2 on tiack 2 (2 min.2g.l-5 s).
However, length L, of track 2 might be
slightfv grearer than
length L, of track .l . How large ca'n Lr_ I],
be tor us still to
conclude that runner 1 is faster? il_w
i:S To
set a speed record
in a
measured (straightJine)
distance d, a race car must be driven first
in one direction (in
time l,) and then in the opposite direction (in
rime rr).(a) To
eliminate the effects of the wind and obtain
th. .;;.;';;;;;;
in a windless situation, should we find the average
ol d/t1 and,
d/t, (method 1) or should we divide d by the
uu".ug" of /r and
tr'? (b) What
is the fractional difference in the two methods
when a steady wind blows arong the car's
route ancr the ratio of
the wind speed r, to the car,s speed u,. is0.0240?
**9 You are to drive to an interview in another =#
town, at a dis_
tance of 300km on an expressway. The
interview is at 11:1-5
A.M. You plan to drive at 100 km/h, so you
leave at g : 00 A.M. to
allow some extra time. you clrive at that speecl
for the first l00
km, but then construction work forces you
to slow to 40 km/h
for 40 km. What would be the least speed neeJeA
for the rest
of the trip to arrive in time for the interview?
selS Panic escape.Figwe 2_22
shows a general situation in
ts-L+ FLi Fz*l
which a stream of people
at_
tempt to escape through an exit
door that turns out to be locked.
It>
The people move toward
door at speed u,
:
the
3.50 m/s. are
Locked
dorlr
FlG.
?-a:
Problem 10.