Regarding the Earth and a cloud layer 800 m above the Earth as the "plates" of a capacitor, calculate the capacitance if the cloud layer has an area of 1.0 km2 . If an electric field of 2.0 × 10^6 N/C makes the air break down and conduct electricity (lightning), what is the maximum charge the cloud can hold?
11.1 nF, 17.7 C
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Energy and Momentum: A baseball pitcher is employing a ballistic pendulum to determine the speed of his fastball. A 3.3-kg lump of clay is suspended from a cord 2.0 m long. When the pitcher throws his fastball aimed directly at the clay, the ball suddenly becomes embedded in the clay and the two swing up to a maximum height of 0.080 m. If the mass of the baseball is 0.21 kg, find the speed of the pitched ball.
Fill in the blank(s) with the appropriate word(s).
Bumpers on cars are not of much use in a collision
To see why, calculate the average force a bumper would have to exert if it brought a 1200-kg car (a so-called compact model) to a rest in 15 cm when the car had an initial speed of 2.0 m/s (about 4.5 mph). (Bumpers are built with springs that compress to provide a stopping force without, hopefully, denting the metal.) A) 1.8 × 104 N B) 1.6 × 104 N C) 5.4 × 104 N D) 6.5 × 105 N E) 3.2 × 104 N
How does the spectrum of a star depend on its surface temperature?
A. The cooler the star, the more molecular absorption bands will be present in its spectrum. B. Hot stars and cool stars can both have weak hydrogen lines. C. The hotter the star, the more colors will be present in its continuous spectrum. D. All of these choices are correct. E. None of these choices are correct.
In order to train for a high-g environment, an astronaut is placed in a centrifuge of radius several meters and rotated in a horizontal circle at constant speed. In this situation, in which direction is the net force on the astronaut?
a. The net force is down since gravity cannot be turned off. b. The net force is horizontal toward the center of the circle. c. The net force is between horizontal toward the center of the circle and straight up, the angle depending on the speed and the radius involved. d. The net force is zero since the astronaut is moving at constant speed and thus the force has no direction.