kuruvila (lk5992) – HW 4 – opyrchal – (11113)
1
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printout
should
have
12
questions.
Multiplechoice questions may continue on
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before answering.
001
10.0 points
A person of mass 88
.
1 kg escapes from a burn
ing building by jumping from a window situ
ated 23
.
9 m above a catching net.
The acceleration of gravity is 9
.
8 m
/
s
2
.
If air resistance exerts a force of 110
.
5 N
on him as he falls, determine his speed just
before he hits the net.
Correct answer: 20
.
2111 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 [(88
.
1 kg)(9
.
8 m
/
s
2
)

110
.
5 N] (23
.
9 m)
88
.
1 kg
= 20
.
2111 m
/
s
.
002
(part 1 of 3) 10.0 points
A boat moves through the water with two
forces acting on it. One is a 2112 N forward
push by the motor on the propeller, and the
other is an 1877 N resistive force due to the
water around the bow.
What is the acceleration of the 1125 kg
boat?
Correct answer: 0
.
208889 m
/
s
2
.
Explanation:
Given :
F
1
= 2112 N
,
F
2
= 1877 N
,
and
m
= 1125 kg
.
2112 N
1877 N
The forces are unbalanced, so the net force
is
F
net
=
m a
=
F
1

F
2
and
a
=
F
1

F
2
m
=
2112 N

1877 N
1125 kg
=
0
.
208889 m
/
s
2
forward.
003
(part 2 of 3) 10.0 points
If it starts from rest, how far will it move in
15
.
5 s?
Correct answer: 25
.
0928 m.
Explanation:
Given :
Δ
t
= 15
.
5 s
Under acceleration,
Δ
x
=
v
i
t
+
1
2
a
(Δ
t
)
2
=
1
2
a
(Δ
t
)
2
since
v
i
= 0 m/s.
Thus
Δ
x
=
1
2
a
(Δ
t
)
2
=
1
2
(
0
.
208889 m
/
s
2
)
(15
.
5 s)
2
=
25
.
0928 m
.
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 Fall '08
 moro
 Force, Mass, General Relativity, Correct Answer, Fnet

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