A rock is thrown upward with a speed of 15 m/s. One second later, a second rock is thrown upward. The second rock hits the first rock when they are both 11 meters above the ground.

A) Why are there two possible speeds at which the second rock can be thrown for these two rocks to collide?
B) Calculate the two speeds of the second rock that will make this happen.

As part of an industrial process, air as an ideal gas at 10 bar, 400k expands at steady state through a valve to a pressure of 4 bar. the mass flow rate of air is 0.5 kg/s. the air then passes through a heat exchanger where it is cooled to a temperature of 295k with negligible change in pressure. the valve can be modeled as a throttling process, and kinetic and potential energy effects can be neglected. (a) for a control volume enclosing the valve and heat exchanger and enough of the local surroundings that the heat transfer occurs at the ambient temperature of 295 k, determine the rate of entropy production, in kw/k. (b) if the expansion valve were replaced by an adiabatic turbine operating isentropically, what would be the entropy production? compare the results of parts (a) and (b) and discuss.

Which is not true about oxygen-17 and oxygen-18?

Atomic nuclei of almost all elements consist of