7) The four devices shown in the figure on the next page rest on frictionless wh

Question

7) The four devices shown in the figure on the next page rest on frictionless wheels, are restricted to move in the x-direction only, and are initially held stationary. The pressure at the inlets and outlets of each is atmospheric, and the flow is incompressible. The contents of each device is not known. When released, which devices will move to the right and which to the left? Explain. (Note: Answers without explanations will not receive credit, Hint: Assume that there is a force holding each device stationary initially. Use the Linear Momentum Equation for a CV to determine the direction of the applied force, and then think about which way the device will move when the force is removed.)

Explanation / Answer

Diagram A moves right side. Because air flow from right side and exits on left side of the passage. The body will move opposite direction to the air flow direction(newtons 3rd law). Air is passing in large inlet area and it moves through reduced area at the bottom side. Reduction area will cause to reduce the pressure and increases the velocity of the air. Diagram B moves no where . Because of the divergent exit area, the air flow velocity is dropped and cannot produce enough thrust to move the body. Diagram C also has no movement. Reason is as same as mentioned in body “B”. And Airflow is in y axis and movement is restricted in y axis. Diagram D moves left side. Air if pass into wide area inlet and exit in smaller exit which produce more velocity and that cause to move the body to opposite direction of air flow.

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