hiarker (srh959) – homework 17 – Turner – (58220)
1
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13
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001
(part 1 of 2) 10.0 points
A 1100 kg car starts from rest and accelerates
uniformly to 12
.
7 m
/
s in 14
.
6 s
.
Assume that air resistance remains con
stant at 361 N during this time.
Find the average power developed by the
engine.
Correct answer: 11
.
2176 hp.
Explanation:
m
= 1100 kg
,
v
i
= 0 m
/
s
,
v
f
= 12
.
7 m
/
s
,
and
Δ
t
= 14
.
6 s
.
The acceleration of the car is
a
=
v
f

v
i
Δ
t
=
v
f
Δ
t
since v
i
= 0, so
a
=
12
.
7 m
/
s
14
.
6 s
= 0
.
869863 m
/
s
2
.
Thus the constant forward force due to the
engine is found from
summationdisplay
F
=
F
engine

F
air
=
m a
F
engine
=
F
air
+
m a
= 361 N + (1100 kg)
(
0
.
869863 m
/
s
2
)
= 1317
.
85 N
.
The average velocity of the car during this
interval is
v
av
=
v
f
+
v
i
2
,
so the average power output is
P
=
F
engine
v
av
=
F
engine
parenleftBig
v
f
2
parenrightBig
= (1317
.
85 N)
parenleftbigg
12
.
7 m
/
s
2
parenrightbigg parenleftbigg
1 hp
764 W
parenrightbigg
=
11
.
2176 hp
.
002
(part 2 of 2) 10.0 points
Find the instantaneous power output of the
engine at
t
= 14
.
6 s just before the car stops
accelerating.
Correct answer: 22
.
4352 hp.
Explanation:
The instantaneous velocity is 12
.
7 m
/
s and
the instantaneous power output of the engine
is
P
=
F
engine
v
f
= (1317
.
85 N)(12
.
7 m
/
s)
parenleftbigg
1 hp
764 W
parenrightbigg
=
22
.
4352 hp
.
003
(part 1 of 2) 10.0 points
Three 2 kg masses are located at points in
the
xy
plane as shown.
43 cm
54 cm
What is the magnitude of the resultant
force (caused by the other two masses) on
the mass at the origin? The universal gravita
tional constant is 6
.
6726
×
10
−
11
N
·
m
2
/
kg
2
.
Correct answer: 1
.
70924
×
10
−
9
N.
Explanation:
Let :
m
= 2 kg
,
x
= 43 cm = 0
.
43 m
,
y
= 54 cm = 0
.
54 m
,
and
G
= 6
.
6726
×
10
−
11
N
·
m
2
/
kg
2
.
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hiarker (srh959) – homework 17 – Turner – (58220)
2
The force from the mass on the right points
in the
x
direction and has magnitude
F
1
=
G
m m
x
2
=
G m
2
x
2
=
(6
.
6726
×
10
−
11
N
·
m
2
/
kg
2
) (2 kg)
2
(0
.
43 m)
2
= 1
.
4435
×
10
−
9
N
.
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 Spring '09
 NUI
 Physics, Force, Mass, Work, General Relativity, Correct Answer, Celestial mechanics

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