# Physic for Engineer

1. Sep 22, 2007

### physicnow

Pls help on this problem that I can not solve it.

a room measures 3.0 m by 4.5 m by 6.0 m. The heating and air conditioning ducts to and from the room are circular with diameter 0.30 m, and the air in the room is to be exchanged every 12 minutes. What is the necessary flow speed in the duct? ( Assume that the density of the air is constant)

thanks

2. Sep 22, 2007

### Staff: Mentor

Please do not post homework problems in the tutorial section.

Basically, the is a volumetric flow rate problem. One knows the total volume of the room (L * W * H) which has to be replaced in 12 minutes or 720 s.

One knows the diameter of the duct work, so one computes the area.

Q = V * A, where Q = volumetric flowrate, V = mean velocity, and A = cross-sectional area.

3. Sep 22, 2007

### physicnow

Thanks

Thank for reply, Astronuc, I have a question, you said Q=V*A, V=velocity, but in the problem, it does not give velocity value, how do I get it. Thanks again.

4. Sep 22, 2007

### Staff: Mentor

V = Q/A, and one must compute Q and A to get V.