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Is the force on a moment/torque carried?

by Illusion3
Tags: carried, force, moment or torque
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Illusion3
#1
Jan28-11, 03:32 AM
P: 4
Hello,

I had an interesting thought about how moments work, and if the force is carried.

If I am pushing on the handle of a door, say 10 metres away from the pivot, a force of 10 newtons, the total moment would be 100nm.

Does this mean that at every point of the door, half way for instance, the moment will be the same? Say for instance at the point 5 metres from the pivot, can you say that the moment at that point is also 100nm, and thus the force must be 50 newtons? This makes no sense, as the force applied 1cm from the pivot would be 1000 newtons.

Albeit what my teacher told me, I believe that if you half the distance from where you push, the force will also be halved, and thus the moment quartered, although I have no mathematical proof.

Thanks for your help!
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tiny-tim
#2
Jan28-11, 04:54 AM
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Hello Illusion3!

i'm not sure i follow you

if the amount of force is fixed, then the torque gets less as you get nearer the hinge

the door is harder to push, in the sense that it moves slower for the same force

alternatively, if you push with the same moment, then the amount of force you need is increased as you get nearer the hinge
Illusion3
#3
Jan28-11, 05:03 AM
P: 4
Thanks for your reply,

I think my question is a little confusing. Please look at the following attachment that should display the diagram about my question.

In other words, the diagram shows that if I apply a force at the top, obviously half way in the middle there is also a moment, although my hand doesn't push at that area. What would the force be half way in the middle?
Attached Images
File Type: bmp Moment.bmp (257.8 KB, 10 views)

tiny-tim
#4
Jan28-11, 05:11 AM
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Is the force on a moment/torque carried?

Quote Quote by Illusion3 View Post
In other words, the diagram shows that if I apply a force at the top, obviously half way in the middle there is also a moment, although my hand doesn't push at that area. What would the force be half way in the middle?
Sorry, I have no idea what you mean.

A moment of what about what?
Femme_physics
#5
Jan28-11, 06:44 AM
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Easy.

If you apply 10 Newtons 10 meters away from the hinge, it's 100 Nm (since you do 10x10)

If you apply 10 Newtons 5 meters away from, it'd be 50 Nm that you're applying (since you do 10x5)

So, the closer you get to the hinge, provided you're applying a constant force, the moment will decrease. That's why you never push a door to open it close to the hinge.


If you want the moment to equal 100 5 meters away from the door, you'd need to apply 20 Newtons (since you do 20x5)

Hope that helps,

-Dory


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