You have a machine which can accelerate pucks on frictionless ice. Starting from rest, the puck travels a distance x in time t when force F is applied. If force 3F is applied, the distance the puck travels in time t is
a. x.
b. (3/2)x.
c. 3x.
d. (9/2)x.
e. 9x.
c
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In grinding a steel knife blade (specific heat = 0.11 cal/g-C°), the metal can get as hot as 400.°C. If the blade has a mass of 80
g, what is the minimum amount of water needed at 20°C if the water is not to rise above the boiling point when the hot blade is quenched in it?
As you walk away from a vertical plane mirror, your image in the mirror
A) is always the same height. B) may or may not decrease in height, depending on where the observer is positioned. C) is always a real image, no matter how far you are from the mirror. D) changes from being a virtual image to a real image as you pass the focal point. E) decreases in height.
The image below shows a rock formation nicknamed “the face on Mars” that was taken by the Viking 1 orbiter in 1976. Given the clear resemblance to a face, what is an appropriate scientific reaction to this image?
A) Assume it is most likely a coincidental result of light and shadow, and plan higher resolution follow-up observations.
B) Assume that the image is probably evidence of an advanced civilization, and develop plans to attempt radio communication wit the civilization.
C) Conclude that the resemblance to a human face is so clear that it cannot have been made by nature and that this geological feature must have been built by an advanced civilization.
D) Assume that NASA's spacecraft must have been hacked by someone perpetrating a hoax, and therefore ignore the image
An electronic circuit contains a power resistor that dissipates 1.5 W. The designer wants to modify the circuitry in such a way that it will be necessary for the resistor to dissipate 2.5 W. The resistor is in the shape of a disk 1 cm in diameter and 0.6-mm-thick. Its surface is aligned with a cooling air flow at 30°C and 10 m/s velocity, i.e., the vertical axis of the disc is in cross-flow with the cooling air. The resistor lifetime becomes unacceptable if its surface temperature exceeds 90°C. Is it necessary to replace the resistor for the new circuit?
GIVEN
• A heat generating resistor disk with its surface aligned with a cooling airflow
• Heat generation rate ( Q
G) = 2.5 W
• Disk diameter (D) = 1 cm = 0.01 m
• Disk thickness (t) = 0.6 mm = 0.0006 m
• Air temperature (T?) = 30°C
• Air velocity (U?) = 10 m/s
• Maximum surface temperature (Ts) = 90°C
FIND
• Is it necessary to replace the resistor?
ASSUMPTIONS
• Steady state
• Radiation is negligible
SKETCH
