johnson (mjj622) – HW 4 – Coker – (56625)
1
This
printout
should
have
24
questions.
Multiplechoice questions may continue on
the next column or page – find all choices
before answering.
This
assignment
covers
the
material
in
Chapter 5, namely Newton’s 2nd and 3rd laws
and their application.
001
10.0 points
Before the time of Galileo and Newton, some
learned scholars thought that a stone dropped
from the top of a tall mast of a moving ship
would fall vertically and hit the deck behind
the mast by a distance equal to how far the
ship had moved forward while the stone was
falling.
In light of your understanding of Newton’s
first law, what is true?
1.
The stone will fall in some trajectory
depending on the speed of the ship.
2.
If the ship speed is fast enough, the stone
will drop into the sea.
3.
The stone will have a horizontal motion;
it will hit the deck in front of the mast.
4.
Everyone on the ship will see the stone
fall vertically if released from rest.
correct
5.
All are wrong.
Explanation:
A stone will fall vertically when released
from rest.
If the stone is dropped from the
top of the mast of a moving ship, the hori
zontal motion is not changed while the stone
drops – providing the air drag on the stone
is negligible and the ship’s motion is steady
and straight. From the frame of reference of
the moving ship, the stone falls in a vertical
straight line path, landing at the base of the
mast.
002
(part 1 of 2) 10.0 points
The position of a toy locomotive moving on a
straight track along the
x
axis is given by the
equation
x
=
b t
3

c t
2
+
d t ,
where
b
= 3 m
/
s
3
,
c
= 189 m
/
s
2
, and
d
=
3969 m
/
s,
x
is in meters and
t
is in seconds.
Find the time
t
when the net force on the
locomotive is equal to zero.
Correct answer: 21 s.
Explanation:
From Newton’s second law of motion the
force is directly proportional to the accelera
tion, which means zero acceleration will give
a zero force.
The velocity is
v
=
d x
dt
= 3
b t
2

2
c t
+
d ,
and the acceleration is
a
=
d
2
x
dt
2
=
d v
dt
= 6
b t

2
c
= 0
,
so
t
=
c
3
b
=
(189 m
/
s
2
)
3 (3 m
/
s
3
)
=
21 s
.
003
(part 2 of 2) 10.0 points
At the time when the net force on the loco
motive is equal to zero, the velocity of the
locomotive is
1.
zero.
correct
2.
positive.
3.
negative.
Explanation:
Since
v
= 0 m
/
s, the velocity is negative,
see Part 1.
004
10.0 points
Consider a toy car on an inclined ramp. The
car is pushed up the ramp and released, it
then coasts up the ramp for a while, slows
down to a complete stop, and eventually rolls
back down the ramp.
This problem focuses
on the time period when the car is coasting
up the ramp, after its released but before it
stops.
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johnson (mjj622) – HW 4 – Coker – (56625)
2
vector
v
Let us neglect the resistive forces such as
friction and air drag and consider the net sum
vector
F
net
of all the other forces.
Which of the
following statements describes the behavior
of this net force
vector
F
net
during the time the car
coasts up the ramp
?
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 Fall '08
 Turner
 Force, Friction, Fnet, Coker

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