Thevalingam, Donald – Homework 15 – Due: Jan 16 2007, midnight – Inst: Eslami
1
This
printout
should
have
16
questions.
Multiplechoice questions may continue on
the next column or page – find all choices
before answering.
The due time is Central
time.
001
(part 1 of 1) 10 points
How far will an object move in 9 s if its average
speed during that time is 40 m
/
s?
Correct answer: 360 m.
Explanation:
If an object is moving during time
t
with
average speed
v
then the distance traveled
will be
s
=
vt
keywords:
002
(part 1 of 1) 10 points
In order to qualify for the finals in a racing
event, a race car must achieve an average
speed of 252 km
/
h on a track with a total
length of 1800 m.
If a particular car covers the first half of the
track at an average speed of 216 km
/
h, what
minimum average speed must it have in the
second half of the event in order to qualify?
Correct answer: 302
.
4 km
/
h.
Explanation:
Let
t
t
be the maximum time to complete
the trip.
t
t
=
total distance
needed average speed
=
d
v
av
The time spent to the first half,
t
1
is
t
1
=
half distance
v
av
1
=
d/
2
v
av
1
Thus, the maximum time that can be spent
second half of the trip is
t
2
=
t
t

t
1
and the required average speed on the second
half is
v
av
2
=
d/
2
t
2
keywords:
003
(part 1 of 3) 10 points
A ball is dropped from rest at point
O
. After
falling for some time, it passes by a window of
height 3
.
1 m and it does so during time 0
.
31 s.
The acceleration of gravity is 9
.
8 m
/
s
2
.
O
A
B
3
.
1 m
x
y
The ball accelerates all the way down; let
v
A
be its speed as it passes the window’s top
A
and
v
B
its speed as it passes the window’s
bottom
B
.
How much did the ball speed up as it passed
the window;
i.e.
, calculate Δ
v
down
=
v
B

v
A
?
Correct answer: 3
.
038 m
/
s.
Explanation:
Let :
h
= 3
.
1 m
,
t
AB
= 0
.
31 s
,
and
g
= 9
.
8 m
/
s
2
.
O
A
B
h
t
AB
t
y
Assume:
Down is positive.
The ball falls under a constant acceleration
g
, so
g
=
Δ
v
t
=
v
B

v
A
t
and the change of its velocity during time
t
AB
is simply Δ
~v
=
~v
B

~v
A
=
g t
AB
, assuming
the downward direction to be positive.
Δ
v
down
= (9
.
8 m
/
s
2
)(0
.
31 s) = 3
.
038 m
/
s
.
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Thevalingam, Donald – Homework 15 – Due: Jan 16 2007, midnight – Inst: Eslami
2
004
(part 2 of 3) 10 points
Calculate the speed
v
A
at which the ball
passes the window’s top.
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