The laws of thermal radiation tell us that a star emits light with a total power per unit area that depends only on the star's surface temperature. the formula is power per unit area=σt4 where σ is a constant with a measured value of σ=5.7×10−8wm2×k4 (as usual, m stands for meters and k stands for kelvins.) you can then find the total power (or luminosity) emitted by the star by multiplying its total surface area by the power per unit area. use these facts to calculate the total power emitted into space by a star with a radius of 8.0×108m and a surface temperature of 6500 k. you will need the formula for the surface area of a sphere, which is surface area (sphere)=4πr2

Search coils and credit cards. one practical way to measure magnetic field strength uses a small, closely wound coil called a search coil. the coil is initially held with its plane perpendicular to a magnetic field. the coil is then either quickly rotated a quarter-turn about a diameter or quickly pulled out of the field. (a) derive the equation relating the total charge q that flows through a search coil to the magnetic-field magnitude b. the search coil has n turns, each with area a, and the flux through the coil is decreased from its initial maximum value to zero in a time ∆t. the resistance of the coil is r, and the total charge is q = i∆t, where i is the average current induced by the change in flux. (b) in a credit card reader, the magnetic strip on the back of a credit card is rapidly “swiped” past a coil within the reader. explain, using the same ideas that underlie the operation of a search coil, how the reader can decode the information stored in the pattern of magnetization on the strip. (c) is it necessary that the credit card be “swiped” through the reader at exactly the right speed? why or why not?

What is the average speed of a car that travels 40 mph for 1 hour and 60 mph in another hour?

Which is not an example of how an object gains elastic potential energy by stretching? a. jumping on a pogo stick b. pulling on a rubber band c. jumping on a trampoline