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Old Oct 24th 2017, 01:08 AM   #1
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Thrust force and reistant forces

Hello

Can anyone help with the problem below?

"A fresh water storage tank in a factory is shown in the figure above. Its top side is denoted as C, the narrow side as B and the long side as A. In order to allow regular maintenance and cleaning side A is hinged at its bottom edge and secured using a heavy-duty clasp at its top edge. L = 1.25m, H = 0.6m & W = 0.5m"

When the tank is totally full of water:

a) Calculate the size and location of the water thrust force on side A
b) Draw a side on view of side A (looking along arrow D) and show the size of the two forces acting on the tank side and how high they are located from the bottom of the tank
c) Calculate the required resisting force at the clasp to keep the panel shut.

I have claculated that the size and location of the water thrust force on side A is 2.2KN located at 0.2m from the bottom edge. My tutor has told me this is correct.
For part b I drew the force of the water perpendicular to the door and the force of the clasp and hinge in the opposite direction located at the top and bottom.
I thought that the resisting force would be equal but opposite in direction to the moment around the hinge. To work that out I multiplied the thrust force by the distance from the location of the force to the hinge. I was told this was incorrect and the incorrect method. So bit stuck now!

Any help would be greatly appreciated.
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Thrust force and reistant forces-q1.png  
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Old Oct 24th 2017, 03:21 PM   #2
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Let H be the hinge force, C be the clamp force and T be the water thrusts force.

Then by horizontal equilibrium

T = C + H

Take moments about a horizontal line on side A at the depth which T acts.

Then T has zero moment about this line.

Thus 0.4C = 0.2H

or 2C = H

can you complete it now?

Last edited by studiot; Oct 24th 2017 at 03:51 PM.
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Old Oct 25th 2017, 03:59 PM   #3
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Still a little confused if I'm honest.

I appeciate that T = C + H

But I'm not certain how you get to 0.4C = 0.2H and therefore 2C = H

Is it because that as there is no moment around the line at the equal value of the thrust force needs to be applied across H and C and it is split according to the distance from the point at which the force acts against the wall?

Thanks for your help
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Old Oct 26th 2017, 07:21 AM   #4
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Originally Posted by Simonamf View Post
Still a little confused if I'm honest.

I appeciate that T = C + H

But I'm not certain how you get to 0.4C = 0.2H and therefore 2C = H

Is it because that as there is no moment around the line at the equal value of the thrust force needs to be applied across H and C and it is split according to the distance from the point at which the force acts against the wall?

Thanks for your help
OK, to fully understand the mechanics here you have to start off thinking in 3 dimensions.

The gate on face A is in equilibrium and 3D equilibrium requires 6 equations to be satisfied.

3Force equations, one along each axis and 3 Moment equations, one about each axis.

By looking at my first diagram we can quickly reduce this to 2 dimensions.

Now forces act at a point along a line and you can see from my first diagram that the hinge and clasp forces do just this.
But the thrust is distributed across the whole face and acts all along line PQ as shown.
We can replace this by an equivalent force T, acting at the centre of line PQ and develop my second diagram, which the one in part b of your question should look like.

This is 2 dimensional and allows us to write the horizontal and moment equations I have already written, using the information from the first part of your question.

Note we are taking moments about the line PQ so we avoid the question of how the pressure force is distributed along it.
(I assume you ar comfortable with taking moments)

Now I am going to ask you about the third axis because if we take moments about the clasp each hinge force has moment about the third axis through the clasp, so can you write the equilibrium equation for that?
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Old Oct 26th 2017, 01:59 PM   #5
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I think so.....moment = force x distance from axis

Therefore there will be no moment around pq as there is no distance to measure to.

However im still not certain how to determine the monet around the hinge and therefore the force of the clasp required to hold it shut. Is this equal to the total resistive force (which is equal to T in opposite direction) x the distance from T?

If so how do I determine the force of the clasp from this.

Apologies for my ignorance but I am new to this and I have found that the course I have been working on does not explain things particularly well.
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Old Oct 26th 2017, 02:39 PM   #6
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Apologies for my ignorance but I am new to this and I have found that the course I have been working on does not explain things particularly well.
No worries and no need to apologise, help is what these forums are all about.


Yes moment = force x distance to the fulcrum or pivot

Note that distance is measured perpendicular to the line of action of the force.

But here is the important business,

A force has a (different) moment about every point in the plane.

and you are entitled to take any of them!


I have taken moments about the line PQ.

You do not need the moment about the hinge, in fact that one presents some theoretical difficulties, so don't try it till you are comfortable with the basics.

Sorry I am travelling at the moment (pun sorry) so can't do a diagram.

Will see what I can do later.
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Old Oct 26th 2017, 03:08 PM   #7
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I understand that the moment is different depending on the distance from the pivot.

So he moment should be force x 0.2m (distance from line to pivot)

I still dont know how from calculating the moment around a point on the line pq I can determine the force on the clasp.

Appreciate your help
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Old Oct 26th 2017, 03:16 PM   #8
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Is the door in equilibrium or is it rotating about any axis?
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Old Oct 26th 2017, 03:38 PM   #9
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At this point itís in equilibrium.

Therefore I take it the resistive force is equal to the moment but in the opposite direction?
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Old Oct 26th 2017, 03:48 PM   #10
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So if the hinge forces are developing a moment about the line PQ, (=0.2H)

What is developing the countermoment to maintain the equilibrium?
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