# Torque calculation of a gate rotating around its pier

1. Mar 28, 2013

### 125cl

Hey guys,

I am new to this forum, I have a problem and I cant get my head around it. so basically, I have a gate that rotates in water about a pier located in the middle (the gate is is 20 m length from the middle pier on both sides, so total length is 40 m). how do i calculate the force required to move the gate and therefore the torque/moments required to rotate the gate. the gate only rotates by 90 degrees. At first, i though i should calculate the drag force which 1/2*p*v2*Cd and then multiply it by the radius of the circle. but am not really sure if i am doing the right thing. I've attached a picture that shows the gate in its initial position and another picture showing the gate in its final rotated position (btw the rotation is about the middle pier) your help is greatly appreciated (ignore the bridge in the picture) :D

Just to add, the gates will be moving in water. the water density if 1000 kg/m3 and the speed of rotation of the gate is 0.1 m/s

Many thanks
Jemmy

Last edited: Mar 28, 2013
2. Mar 28, 2013

### Mandelbroth

Hi! Welcome to Physics Forums. Two things:

First, it is unclear what you mean. You gave a picture which clearly shows two rotating gates, but then your post says rotation is about the middle pier. In maths we call this a contradiction and slap a Halmos on it. However, I like brain teasers, so I'd like you to clarify what you mean.

Secondly, you have not defined the height of the gate or the height of the water. It is also unclear if you mean tangential speed by speed of rotation.

3. Mar 28, 2013

### 125cl

Hey Mandelbroth,

The rotation is basically around the middle pier of the gates themselves not the huge middle pivot. basically I want to install a motor that can provide enough torque to rotate these gates, so i basically need to find the torque/moment needed to rotate those gates from the middle pier of the gates, my calculation is basically finding the drag forces which perpendicular to the plane of the gate and ive assumed the force act at mid-length of each of the gates, therefore to find the torque ive multiplied the drag force by the half the radius basically.

The height of the gates is 15.6m and lets say water level is at 15.6m as well so all the gate is submerged in water. and by speed, i mean the speed of the movement of the gate around its circular circumference . sorry its just a bit hard to explain it in words.