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le (txl65) – Newton’s Law – mohanty – (81440)
1
This printout should have 16 questions.
Multiplechoice questions may continue on
the next column or page – fnd all choices
beFore answering.
001
10.0 points
A person oF mass 60
.
4 kg escapes From a burn
ing building by jumping From a window situ
ated 27
.
6 m above a catching net.
The acceleration oF gravity is 9
.
8 m
/
s
2
.
IF air resistance exerts a Force oF 79 N on
him as he Falls, determine his speed just beFore
he hits the net.
Correct answer: 21
.
6509 m
/
s.
Explanation:
The Forces acting on him are the gravita
tional Force
mg
acting downward, and the air
resistance,
F
a
, acting upward. The accelera
tion is downward and the net Force is
F
net
=
ma
=
mg

F
a
a
=
mg

F
a
m
The person is in Free Fall, so his fnal speed is
defned by
v
f
=
√
2
a h
=
r
2 (
mg

F
a
)
h
m
=
R
2 [(60
.
4 kg)(9
.
8 m
/
s
2
)

79 N] (27
.
6 m)
60
.
4 kg
= 21
.
6509 m
/
s
.
002
10.0 points
A spider oF mass
m
is sitting in a perFectly
symmetric web (in a vertical plane with 45
◦
angles between the web lines) as shown in the
fgure below, where
T
i
’s denote the tensions
along the lines which are connected to the spi
der at the center with
i
= 1
,
2
,
3
,
4
,
5
,
6
,
7
,
8
.
The tension are all positive.
T
1
T
2
T
3
T
4
T
5
T
6
T
7
T
8
m
How is the tension
T
3
related to
T
2
, T
8
,
and
T
7
?
Hint:
±rom leFtright symmetry, we see that
T
1
=
T
5
, T
2
=
T
4
,
and
T
8
=
T
6
.
1.
T
3
=
T
7
+ (
T
8
+
T
2
)
√
2 +
mg
2.
T
3
=
T
7

(
T
8

T
2
)
√
2

mg
3.
T
3
=
T
7

(
T
8
+
T
2
)
√
2

mg
4.
T
3
=
T
7

(
T
8

T
2
)
√
2 +
mg
5.
T
3
=
T
7
+ (
T
8

T
2
)
√
2 +
mg
correct
6.
T
3
=
T
7
+
mg
7.
T
3
=
T
7
+ (
T
8

T
2
)
√
2

mg
8.
T
3
=
T
7
9.
T
3
=
T
7
+ (
T
8
+
T
2
)
√
2

mg
10.
T
3
=
T
7

(
T
8
+
T
2
)
√
2 +
mg
Explanation:
Basic Concept
Newton’s 2nd Law
Solution:
In the
x
(horizontal) direction, we
have
(
T
2
+
T
8
)
1
√
2
+
T
1

(
T
4
+
T
6
)
1
√
2

T
5
= 0
,
so,
T
1
=
T
5
, T
2
=
T
4
,
and
T
8
=
T
6
.
In the
y
(vertical) direction, we have
(
T
2
+
T
4
)
1
√
2
+
T
3

(
T
8
+
T
6
)
1
√
2

T
7

mg
= 0
,
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2
and using the condition
T
2
=
T
4
and
T
8
=
T
6
,
T
2
√
2 +
T
3

T
8
√
2

T
7

mg
= 0
,
or
T
3
=
T
7
+ (
T
8

T
2
)
√
2 +
mg .
003
10.0 points
Beth, a construction worker, attempts to pull
a stake out of the ground by pulling on a rope
that is attached to the stake. The rope makes
an angle of 68
.
9
◦
with the horizontal.
If Beth exerts a force of 133 N on the rope,
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This note was uploaded on 04/05/2011 for the course SD ds taught by Professor Sdsd during the Spring '11 term at Tarrant County.
 Spring '11
 sdsd

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