The force of friction on an object is dependent on two things. what are they?

Which of the following is not am example of a polymer? protein nylon kevlar concreta

Wavelength,frequency,and energy are related.what happens to a wave as it’s wavelength gets shorter?

Air is to be preheated by hot exhaust gases in a cross-flow heat exchanger before it enters the furnace. air enters the heat exchanger at 95 kpa and 20°c at a rate of 0.6 m^3/s. the combustion gases (cp = 1.10 kj/kg°c) enter at 160°c at a rate of 0.95 kg/s and leave at 95°c. determine the rate of heat transfer to the air and its outlet temperature.