Transcript File

Honors Physics
Chapter 5
End-Of-Chapter
Discussion Questions
Page 199
28 November 2K + 4
Directions
Most of the questions have the
answer following the question.
Read the question, give it some
thought and then read the answer.
The questions will not be discussed
in class unless you ask for further
clarification.
1. How would you find the mass
of an object in interstellar
space, where the force of
gravity approaches zero?
P 199
1A. You could not use a balance or
spring scale to find the mass of
the object since both depend on
gravity to operate (gravitational
mass). You would have to
measure the inertial mass of the
object. That is, you would have
to measure the mass using the
relationship F = ma. If a known
force is applied to the object, it
will accelerate. By measuring the
resulting acceleration and
substituting into F = ma, the
mass could be determined.
2. Why is it usually incorrect to
say that an astronaut is
“weightless” as he moves in
orbit above the Earth?
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2A. Weight is defined as the force of
gravity on a mass. The astronaut is
still in the earth's gravitational field
while in orbit. That is, the force of
gravity is acting on the astronaut and
there must be a weight. As explained
in the text, the astronaut's mass may
not be supported by the space
capsule and an apparent weightless
condition exists but this does not
mean that the astronaut has no
weight.
3. A girl hopes to meet her boyfriend
by sliding down a rope, made of
nylon stockings, from her second
story bedroom window. The
stockings will break if the tension
in them exceeds 400 N. The mass
of the girl is 60 kg. Describe how
she should slide down the rope so
that she will land in the arms of
her beloved at a minimum velocity.
a = -3.1 m/s2
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3A. Draw a free body diagram of the girl.
Two forces act on her: gravity down
with a force of (60 kg)(9.8 N/kg) = 588
N and the stocking pulls up with a
tension T. If she had no acceleration,
then T would be 588 N. which is
greater than the breaking strength of
the stocking. So she must decrease
the tension to 400 N by climbing
down the stocking or letting it slide
through her hands. Her minimum
downward acceleration would be
given by:
4. What effect does the rotation
of the Earth have on the
apparent force of gravity on a
man standing (a) at the
equator and (b) at the North
Pole?
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4A. At the equator the force of gravity is
both attracting the man to the earth
and keeping him moving in a circular
path at approximately 1670 km/h. As
a result, the force holding him away
from the earth, as measured on a
bathroom scale, would be slightly
less than that at the pole where there
is no centripetal acceleration. Again,
a free body diagram of the man at the
equator and at the pole will illustrate
the difference. Approx 0.3%
5. A baseball is thrown vertically
into the air. If there were no air
resistance to be considered, it
should return to its original
position with the same speed
as it had when thrown. But,
given that air resistance is a
factor, how will this affect its
speed? Explain your
reasoning.
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5A. Frictional forces always act in the
opposite direction to that of the
motion. At every point on the way up,
the forces of gravity and air
resistance both act downwards.
However, at every point on the way
down, the direction of the air
resistance is up, opposite to the
force of gravity. Hence, the
acceleration for the downwards
section of the motion is smaller in
magnitude than for the upwards
section and the final speed will be
less than the initial speed.
6. In a disaster film, an elevator
full of people falls freely
when the cable snaps. In the
film, the people are shown
pressed upward against the
ceiling. Is this good physics?
Explain.
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6A. This is bad physics. The people
should be shown in random
positions much like astronauts
in an orbiting space capsule. The
only way they could all be at the
top of the elevator is if they
pushed up from the floor when
the cable snapped. Otherwise,
the elevator would have to be
accelerating downward with an
acceleration greater than g,
which is nearly impossible.
7. Future space stations may be
designed like a giant wheel
rotating about a central axis.
The astronauts would live
along the circumference of
this structure. How would
such a structure simulate
gravity?
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7A. On the inside rim of the rotating
space station, the surface would
exert a centripetal force on any
object. This force would act on the
object producing an apparent weight
much the same as is produced by
gravity on the earth. The period of
rotation would determine the value of
g. It would be set lower than that of
earth (9-8 N/kg) to make movement
easier. Note that the larger the
diameter of the space station, the
slower it must turn to achieve the
same artificial gravity.
8. You wish to shoot a rocket to a
point due north of your
present position. In what
direction should you aim the
rocket if you are in the
Northern Hemisphere? In the
Southern Hemisphere?
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8A. In the northern hemisphere,
the target moves cast more
slowly than the launch site,
so one must aim west. In the
southern hemisphere, the
target moves east more
quickly than the launch site,
so one must aim east.
9. A car being driven along a mountain
road proceeds through an “S”
curve. The first part of the curve
has twice the radius of the second
(reverse) part. How does the
centripetal force acting on the car in
the first part compare with that
acting on the car in the second
part? If the car doubles its speed,
what changes occur in the size of
the centripetal force acting on the
car as it traverses each part of the
curve?
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9A. The centripetal force is inversely
proportional to the radius and directly
proportional to the square of the speed.
Thus, the centripetal force in the second
part of the curve will be twice that of the
first if the speed remains constant. If the
speed of the car doubles the centripetal
force must be four times greater in the
first section and eight times greater in
the second compared to the first
section at, the original speed. The
centripetal force is provided by the
friction between the tire and the road so
it is quite likely that the car will not
complete the second part of the "S"
turn.
10. A ball rolls off a horizontal table with a
horizontal speed of 2 m/s.
Stroboscopic photographs of the
motion of the ball are taken from the
three positions A, B, and C. In position
A, the camera is in front of the table
with the ball rolling towards it. In
position B, the camera is above the
edge of the table. in position C, the
camera is to one side of the table at
right angles to the plane of the ball's
path. Sketch the picture you would
expect to obtain in each of the three
positions.
P 199
11. A white disc is placed near the
outside edge of a phonograph
turntable that is rotating at 33 ⅓
rev/min. The turntable is on a cart
moving at a constant velocity. The
disc is illuminated by a strobe light
flashing at a constant frequency.
Draw a sketch showing the white
disc as viewed from a fixed point
above the path of the moving cart.
Assume several strobe flashes per
turntable rotation. Consider
various speeds for the cart.
11. F o r Va = vr where va is the
speed of the axle and vr is the
linear speed at a point on the
rim, with respect to the axle.
P 200
12. Very rapid circular motion,
particularly of machinery near
human beings, presents a
serious safety hazard. Describe
in three examples - one each
from your home, the family
automobile, and a family
member's place of work - the
hazard itself, the related physics,
and the precautions and/or
protection required.
P 200
13. Centrifuges assist analysts in
determining the components
of many substances and
separating them out.
Describe two applications in
the following areas: blood
analysis, research into DNA
or proteins, dairy products,
and minerals.
P 200