A paper placed in between two books can be quickly pulled out without moving the books. Which statement explains this phenomenon? A.
There are no forces acting between the books and the paper.
B.
The pull of gravity on the book is greater than the force of gravity on the paper.
C.
The two books exert force only on each other and not on the paper.
D.
The books have a great deal of inertia and do not move easily.

Athin electrical heater is inserted between a long circular rod and a concentric tube with inner and outer radii of 20 and 40 mm. the rod (a) has a thermal conductivity of ka = 0.15 w/(m*k), while the tube (b) has a thermal conductivity of kb = 0.8 w/(m*k) and its outer surface is subjected to convection with a fluid of temperature t= -15°c and heat transfer coefficient 60 w/(m2 *k). the thermal contact resistance between the cylinder surfaces and the heater is negligible. (a) determine the electrical power per unit length of the cylinders (w/m) that is required to maintain the outer surface of cylinder b at 15°c. (b) what is the temperature at the center of cylinder a?