An object is moving forward with a constant velocity. Which statement about this object MUST be true?
A) The net force on the object is zero.
B) The net force on the object is in the forward direction.
C) No forces are acting on the object.
D) The acceleration of the object is in the forward direction.
Answer: A
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The half-life of cobalt-60 is 5.3 years, while that of strontium-90 is about 29 years
Suppose you have samples of both isotopes, and that they initially contain equal numbers of atoms of these isotopes. How will the activities (number of decays per second) of the samples compare? A) The activities will be equal. B) The activity of the cobalt-60 sample will be greater. C) The activity of the strontium-90 sample will be greater. D) The activities cannot be compared without more information.
Making a laser work depends on creating a population inversion, meaning a situation in which:
a. most electrons are in the lower energy state. b. most photons are in the lower energy state. c. more electrons are in a high-energy than a low-energy state. d. the directions of electron motion are reversed, or inverted. e. more photons are in a high-energy than a low-energy state.
Which of the following explains why, about 13,000 years from now, Vega will be a better North Star than Polaris?
A) Earth rotates once each day. B) Earth orbits the Sun once each year. C) The direction of Earth's axis in space precesses with a period of about 26,000 years. D) Stars appear to move randomly relative to our Sun in the local solar neighborhood. E) The universe is expanding.
An electrical transmission line of 1.2 cm diameter carries a current of 200 Amps and has a resistance of 3 × 10–4 ohm per meter of length. If the air around this line is at 16°C, determine the surface temperature on a windy day, assuming a wind blows across the line at 33 km/h.
GIVEN
• An electrical transmission line on a windy day
• Line outside diameter (D) = 12 cm = 0.012 m
• Current (I) = 200b Amps
• Resistance per unit length (Re/L) = 3 × 10–4 ohm/m
• Air temperature (Tb) = 16°C
• Air velocity (V) = 33 km/h = 9.17 m/s
FIND
• The line surface temperature (Ts) ASSUMPTIONS
• Steady state conditions
• Air flow approaching line has low free-stream turbulence
SKETCH
PROPERTIES AND CONSTANTS
Thermal conductivity (k) = 0.0248 W/(m K) Kinematic viscosity (?) = 15.3 × 10–6 m2/s Prandtl number (Pr) = 0.71