An astronaut travels to a star system 4.5 ly away at a speed of 0.90 c . Assume that the time needed to accelerate and accelerate is negligible. a. How long does the journey lake according to Mission Control on earth? b. How long does the journey take according to the astronaut? c. How much time elapses between the launch and the arrival of the first radio message from the astronaut saying that she has arrived?
An astronaut travels to a star system 4.5 ly away at a speed of 0.90 c . Assume that the time needed to accelerate and accelerate is negligible. a. How long does the journey lake according to Mission Control on earth? b. How long does the journey take according to the astronaut? c. How much time elapses between the launch and the arrival of the first radio message from the astronaut saying that she has arrived?
5.77 A block with mass m₁ is placed on an inclined plane with
slope angle a and is connected to a hanging block with mass m₂ by a
cord passing over a small, frictionless pulley (Fig. P5.74). The coef-
ficient of static friction is μs, and the coefficient of kinetic friction is
Mk. (a) Find the value of m₂ for which the block of mass m₁ moves up
the plane at constant speed once it is set in motion. (b) Find the value
of m2 for which the block of mass m₁ moves down the plane at constant
speed once it is set in motion. (c) For what range of values of m₂ will
the blocks remain at rest if they are released from rest?
5.78 .. DATA BIO The Flying Leap of a Flea. High-speed motion
pictures (3500 frames/second) of a jumping 210 μg flea yielded the
data to plot the flea's acceleration as a function of time, as shown in
Fig. P5.78. (See "The Flying Leap of the Flea," by M. Rothschild et al.,
Scientific American, November 1973.) This flea was about 2 mm long
and jumped at a nearly vertical takeoff angle. Using the graph, (a) find
the initial net external force on the flea. How does it compare to the
flea's weight? (b) Find the maximum net external force on this jump-
ing flea. When does this maximum force occur? (c) Use the graph to
find the flea's maximum speed.
Figure P5.78
150
a/g
100
50
1.0
1.5
0.5
Time (ms)
5.4 ⚫ BIO Injuries to the Spinal Column. In the treatment of
spine injuries, it is often necessary to provide tension along the spi-
nal column to stretch the backbone. One device for doing this is the
Stryker frame (Fig. E5.4a, next page). A weight W is attached to
the patient (sometimes around a neck collar, Fig. E5.4b), and fric-
tion between the person's body and the bed prevents sliding. (a) If
the coefficient of static friction between a 78.5 kg patient's body and
the bed is 0.75, what is the maximum traction force along the spi-
nal column that W can provide without causing the patient to slide?
(b) Under the conditions of maximum traction, what is the tension in
each cable attached to the neck collar?
Figure E5.4
(a)
(b)
W
65°
65°
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