Thevalingam, Donald – Homework 25 – Due: Apr 3 2007, midnight – Inst: Eslami
1
This
printout
should
have
10
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
A force of 2080 N is needed to bring a car
moving at 25
.
2 m
/
s to a halt in 18
.
3 s.
What is the mass of the car?
Correct answer: 1510
.
48 kg.
Explanation:
Basic Concepts:
Impulsemomentum re
lationship
m
Δ
v
=
F
Δ
t .
Solution:
The car is brought to a halt, so
Δ
v
= 0

v
=

v
The force is negative, so
m
(

v
) = (

F
)
t
m
=

F t

v
=
F t
v
.
keywords:
002
(part 1 of 1) 10 points
Small rockets are used to make small adjust
ments in the speed of satellites.
One such
rocket has a thrust of 46 N.
If it is fired to change the velocity of a(n)
82000 kg space craft by 66 cm
/
s, how long
should it be fired?
Correct answer: 1176
.
52 s.
Explanation:
Impulse is defined by
I
=
F t
=
m
Δ
v
so
t
=
m
Δ
v
F
Dimensional analysis:
for
t
kg
·
cm
/
s
N
·
1 m
100 cm
=
kg
·
m
/
s
kg
·
m
/
s
2
= s
keywords:
003
(part 1 of 2) 10 points
A revolutionary war cannon, with a mass of
2120 kg, fires a 22
.
5 kg ball horizontally. The
cannonball has a speed of 116 m
/
s after it has
left the barrel.
The cannon carriage is on a
flat platform and is free to roll horizontally.
What is the speed of the cannon immedi
ately after it was fired?
Correct answer: 1
.
23113 m
/
s.
Explanation:
Let :
m
= 22
.
5 kg
,
M
= 2120 kg
,
and
v
= 116 m
/
s
.
The cannon’s velocity immediately after it
was fired is found by using conservation of
momentum along the horizontal direction:
M V
+
m v
= 0
⇒ 
V
=
m
M
v
where
M
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 Spring '08
 Turner
 Physics, Force, Kinetic Energy, Mass, Work, Velocity, m/s, Thevalingam

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