What is an echo, and why is it weaker than the original sound?
What will be an ideal response?
Answer: An echo is reflected sound. Because sound from a small source spreads out via the inverse-square law, sound intensity per unit area decreases with distance to the reflecting surface, and when reflected continues to decrease with distance. Hence, the echo you hear is weaker than the original sound.
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Impulse: A very light ping-pong ball moving east at a speed of 4 m/s collides with a very heavy stationary bowling ball. The Ping-Pong ball bounces back to the west, and the bowling ball moves very slowly to the east. Which object experiences the greater magnitude impulse during the collision?
A. Neither; both experienced the same magnitude impulse. B. the Ping-Pong ball C. the bowling ball D. It is impossible to tell since the actual mass values are not given. E. It is impossible to tell since the velocities after the collision are unknown.
Atmospheric air at 10°C enters a 2 m long smooth rectangular duct with a 7.5 cm x 15 cm cross-section. The mass flow rate of the air is 0.1 kg/s. If the sides are at 150°C, estimate (a) the heat transfer coefficient, (b) the air outlet temperature, (c) the rate of heat transfer, and (d) the pressure drop.
GIVEN
Atmospheric air flow through a rectangular duct
Inlet bulk temperature (Tb,in) = 10°C
Duct length (L) = 2 m
Cross-section = 7.5 cm x 15 cm = 0.075 m ? 0.15 m
Mass flow rate (m) = 0.1 kg/s
Duct surface temperature (Ts) = 150°C
FIND
(a) The heat transfer coefficient ch
(b) The air outlet temperature (Tb,out)
(c) The rate of heat transfer (q)
(d) The pressure drop (?p)
ASSUMPTIONS
Steady state
The duct is smooth
SKETCH
for dry air at the inlet bulk temperature of 20°C
Thermal conductivity (k) = 0.0251 W/(m K)
Absolute viscosity (?) = 18,240 × 10–6 (Ns)/m2
Prandtl number (Pr) = 0.71
Specific heat (c) = 1012 J/(kg K)
Assume you are a particle in a box called the universe. Estimate your minimum kinetic energy. (Assume the "edge" of the box is at a distance of 20. Giga Light Years.)
What will be an ideal response?
Which portion of the electromagnetic spectrum is used to identify fluorescent minerals?
a. ultraviolet light b. X rays c. infrared waves d. gamma rays