Anderson, Matthew – Homework 4 – Due: Oct 3 2007, 11:00 pm – Inst: Opyrchal, H
1
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printout
should
have
9
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
An elevator starts from rest with a constant
upward acceleration and moves 1 m in the first
1
.
7 s. A passenger in the elevator is holding a
8
.
9 kg bundle at the end of a vertical cord.
The acceleration of gravity is 9
.
8 m
/
s
2
.
What is the tension in the cord as the ele
vator accelerates?
Correct answer: 93
.
3792 N.
Explanation:
T
mg
a
elevator
g
Let
h
be the distance traveled and
a
the
acceleration of the elevator. With the initial
velocity being zero, we simplify the following
expression and solve for acceleration of the
elevator:
h
=
v
0
t
+
1
2
a t
2
=
1
2
a t
2
=
⇒
a
=
2
h
t
2
.
The equation describing the forces acting on
the bundle is
F
net
=
m a
=
T

m g
T
=
m
(
g
+
a
)
=
m
g
+
2
h
t
2
¶
= (8
.
9 kg)
•
9
.
8 m
/
s
2
+
2 (1 m)
(1
.
7 s)
2
‚
= 93
.
3792 N
.
keywords:
002
(part 1 of 1) 10 points
Two forces are the only forces acting on a
2
.
4 kg object which moves with an accelera
tion of 2
.
1 m
/
s
2
in the positive
y
direction.
One of the forces acts in the positive
x
direc
tion and has a magnitude of 5
.
1 N.
What is the magnitude of the other force
f
2
?
Correct answer: 7
.
17019 N.
Explanation:
Basic Concepts:
X
F
=
m
a
Solution:
f
1
f
2
f
~
f
2
is the hypotenuse of a right triangle, so
~
f
2
=
q
f
2
+
f
1
2
keywords:
003
(part 1 of 1) 10 points
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 Fall '10
 JANOW
 Force, Work, Correct Answer

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