sela (ds39632) – HW04 – janow – (11196)
1
This
printout
should
have
11
questions.
Multiplechoice questions may continue on
the next column or page – find all choices
before answering.
001
10.0points
A person of mass 96
.
3 kg escapes from a burn
ing building by jumping from a window situ
ated 24
.
7 m above a catching net.
The acceleration of gravity is 9
.
8 m
/
s
2
.
If air resistance exerts a force of 89
.
2 N
on him as he falls, determine his speed just
before he hits the net.
Correct answer: 20
.
9371 m
/
s.
Explanation:
The forces acting on him are the gravita
tional force
m g
acting downward, and the air
resistance,
F
a
, acting upward. The accelera
tion is downward and the net force is
F
net
=
m a
=
m g
−
F
a
a
=
m g
−
F
a
m
The person is in free fall, so his final speed is
defined by
v
f
=
√
2
a h
=
radicalbigg
2 (
m g
−
F
a
)
h
m
=
radicalBigg
2 [(96
.
3 kg)(9
.
8 m
/
s
2
)
−
89
.
2 N] (24
.
7 m)
96
.
3 kg
= 20
.
9371 m
/
s
.
002
10.0points
A 4000 kg helicopter accelerates upward at
3
.
19 m
/
s
2
.
The acceleration of gravity is 9
.
8 m
/
s
2
.
What lift force is exerted by the air on the
propellers?
Correct answer: 51960 N.
Explanation:
The propellers exert a force F upward and
gravity acts downward.
The net force (di
rected upward) on the helicopter is
F
=
m a
=
summationdisplay
F
up
−
summationdisplay
F
down
=
F
−
m g
Thus
F
=
m a
+
m g
=
m
(
a
+
g
)
Note:
Here
a
and
g
are absolute values of the
helicopter’s acceleration and the acceleration
of gravity, respectively.
003(part1of3)10.0points
A 3
.
1 kg object is subjected to two forces,
vector
F
1
= (1
.
4 N) ˆ
ı
+ (
−
1
.
2 N) ˆ
and
vector
F
2
=
(4
.
4 N) ˆ
ı
+ (
−
11
.
5 N) ˆ
.
The object is at
rest at the origin at time
t
= 0.
What is the magnitude of the object’s ac
celeration?
Correct answer: 4
.
50378 m
/
s
2
.
Explanation:
Let :
vector
F
1
= (1
.
4 N) ˆ
ı
+ (
−
1
.
2 N) ˆ
,
vector
F
2
= (4
.
4 N) ˆ
ı
+ (
−
11
.
5 N) ˆ
,
and
vectorv
o
= 0 m
/
s
.
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 Spring '08
 moro
 Force, Mass, General Relativity, Correct Answer, SelA

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