Nora tied a string around a tennis ball, and then she swung it in a circle in front of her to demonstrate a planet orbiting the sun. she explained that her hand represented the sun, the ball represented a planet, and the string kept the ball from leaving the "orbit" around her hand. why doesn't the string work in the same way as gravity? a. the string holding the ball in orbit is a contact force; gravity is a noncontact force. b. the string only pulls one way; gravity pulls both ways. c. the string keeps the distance between the bodies the same; gravity makes the distance vary. d. there is no difference between the two systems.

Which statement best describes how private industry decides where to spend the money on research and development

How many molecules are present in 3 mols of silicon dioxide (sio2)?

Comedians like to joke that the reason we haven’t been visited by intelligent life from elsewhere in the universe is that aliens have been monitoring earth’s broadcasts of intellectually embarrassing tv programs, like gilligan’s island, fear factor, the jersey shore, and the jerry springer showand so consider us far too primitive to merit a visit. let’s check the assertion that aliens could have been receiving them. tv programs are broadcast at a frequency of about 100mhz with about 100kw of total power in 30frames per second, which emanatesroughly uniformly in all directions. assume that interstellar space transmits these broadcasts without attenuation. no matter how smart they are, aliens would require at least one photon per frame to interpret our signals. findthe number of photons per unit time per unit area reaching a receiver on a possible planet in the nearest star system, which is about 4 light-years away (a light-year is the distance light travels in a year). if aliens aimed a receiver or detector directly at earth, how big (in diameter) would it have to be to receive a photon per frame