F.4 Phy mechanics (concept)

2014-01-12 11:38 pm

A couple forms when two equal and opposite parallel forces apple to the same body and do not act in the same line.

But ,
If two forces F1,F2 cause the same object to rotate clockwise and anticlockwise
respectively at the same time is a couple ?(such that 2 forces are in the left of the
fixed and rotational centre)

the two forces do not violate the definition of "couple" ,the are equal in magnitude,
opposite in direction ,parallel,act on same object,do not act in the same line.They
just do not cause the object to rotate in the same direction.

回答 (1)

天同

2014-01-13 1:30 am

✔ 最佳答案

The crucial point lies on the fact that you have already had a "fixed rotational centre", or a pivot.

The concept of couple is applied to a freely rotated body. That said, there is no pre-fixed pivot on it. If there was already a pivot on the body, the pivot would provide another force that counter-balance the rotation caused by the couple.

If the body is complelely free to rotate, when a couple is applied to it. It rotates about a point on the line joining the points of action of the two forces.

In your quoted example, if the two parallel and opposite forces are acting to the left of a fixed pivot, it tends to make the body to rotate about a point (say, point P) between the two forces in the clockwise direction. As such, the fixed pivot (which is to the right of the couple, or to the right of point P) would be forced to move in the cloclwise direction. There would then arise a force on the pivot if it is fixed, to produce an anticlockwise moment about point P. As a result, the body remains at rest without any rotation.

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