Physics Help Forum translation/rotation

 May 13th 2014, 04:17 PM #1 Junior Member   Join Date: May 2014 Posts: 1 translation/rotation Am I right to say that a force applied to a lever will create a translation as well as a rotation? (Which is why all forces can be come couple/force systems) So if you figure out the moment of force around the pivot point you can't just ignore the newtons that create that moment?
May 14th 2014, 05:44 AM   #2
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 Originally Posted by ajpik Am I right to say that a force applied to a lever will create a translation as well as a rotation?
No - applying a force to a lever creates a moment about the pivot point, and whatever object is on the other end of the lever rotates about the pivot point. There is no linear translation- assuming of course that the pivot is held stationery.

 Originally Posted by ajpik (Which is why all forces can be come couple/force systems)
This is a different issue. The system consisting of a force applied at the end of the lever and a counteracting force at the pivot is mathemetaically the same as a couple at the pivot, at least in terms of the reslting motion of the systsm. Balance of forces can tell you what the reaction force at the pivot is, given the force applied to the lever and the mass of the object at the other end.

 Originally Posted by ajpik So if you figure out the moment of force around the pivot point you can't just ignore the newtons that create that moment?
To determine the moment about the pivot point you must know how many newtons were applied to create that moment - if that's what you're getting at.

Last edited by ChipB; May 14th 2014 at 06:44 AM.

 May 14th 2014, 11:35 AM #3 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 Care must be taken with Forces, Moments and Couples. A Couple is inherently defined about the rotation axis, a Moment is not. The Moment due to a Force can be derived for any arbitary position. A moment reference point has to be defined at some convienent location. The value of the moment will vary depending on where the moment reference point is chosen to be (relative to the point of action of the force). The Couple and the Moment will be the same if the moment reference point is chosen to be at the centre of rotation. The first part of your post implies a clear pivot point at one end of the lever and the force acting at the other end of the lever. Note that this also imples a second force, at the pivot point, to hold the pivot in place (ie to prevent translation of the "system"). If the pivot is not held in place then the system will translate, The rotatation of the system will depend on the position of the action of the force relative to the centre of mass of the system.

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