# Fluid Mechanics - Drag (Bending Moment)

1. Apr 3, 2013

### MrWinesy

A television transmitting antenna is placed on top of a mast which is 30m high and 300mm in diameter. The mast is on top of a tall building. The wind blows at 35m/s and at normal atmospheric pressure (1.225kg/m^3). If the drag coefficient for the mast is 0.2. calculate:

(i) the total drag force on the mast.
(ii) the bending moment at the base of the mast.

(i) Fd=.5*Cd*ρ*U^2*A=.5*.2*1.225*35^2*(30*0.3)=1350.6N

(ii) but how do you do this question?

(the answer is M=20.26kNm but i do not know how to get there)

2. Apr 3, 2013

### timthereaper

I think they're assuming the drag force is an even distributed load over the entire mast. With that as the case, you can place the 1350.6 N force at the midpoint of the mast (15m). The FBD yields 1350.6 N * 15m = 20259 Nm.

3. Apr 3, 2013

### MrWinesy

that was potentially the most stupid question i have ever asked. thanks for your help sir.

4. Apr 3, 2013

### timthereaper

Hahaha you're welcome. Glad I could help.