Causing rotation of an object

[Thorium]

Just a little question that has been bugging me, lets say we have a rod, and we have some kind of thrust on one end of it perpendicular to the rod, it will spin about its center of mass right? Having another source of thrust on the other side will make it rotate faster but only one will still get it to rotate...I think. Because 1 side is accelerating and the other is not, or am I really confused here.

[goodyhi11]

me too

[Ophiolite]

You don't seem to have fully defined the problem. I shall assume it is resting motionless on a flat surface. The point about which it will rotate would surely depend upon the magnitude of the force, the mass of the rod and the co-efficients of static and dynamic friction?

[Thorium]

Ah sorry, lets assume its in interstellar space completely at rest with no other forces acting on it.

[5614]

this still applies:

"would surely depend upon the magnitude of the force, the mass of the rod and the co-efficients of static and dynamic friction?"

 

if you put a mega rocket on one end of a tiny stick the whole thing would just shoot forward in a near-straight line as the mega rocket wouldnt 'see' the little rod as a resistive force due to its relative small size.

 

if you put two equal rockets on either side of a rod, assuming the rockets are the same, the stick is equal the whole way along and there are no other resistive forces then it'd go in a circle on the spot around the center of the stick (equidistant from the two rockets).

when saying rockets in this post i am referring to the cause of thrust

[swansont]

You will cause rotation because there is a torque, but you may also cause center-of-mass motion. It depends on the specifics of the problem.

[Thorium]

Ok, thanks, just wanted to make sure. So one source of thrust can cause both rotational and translational thrust in this case. I was told without gravity it would only be translational by a misinformed friend in an argument, and there were no specifics given. But perhaps he did mean with a very large amount of thrust as you said 5614. Thanks again

[Edward Duffy]

Ok, thanks, just wanted to make sure. So one source of thrust can cause both rotational and translational thrust in this case. I was told without gravity it would only be translational by a misinformed friend in an argument, and there were no specifics given. But perhaps he did mean with a very large amount of thrust as you said 5614. Thanks again

 

I think to cause rotation the thrust would have to be applied off center, causing the force to be transferred around the center of mass of the rod rather than straight through it. In the case of planetary rotation I think the Stirling engine offers some good clues. When the mass contained within a sphere is heated more on one side than another it migrates from hot side to cool side around the dense center which acts as a heat capacitor. The flowing mass within causes the whole sphere to rotate.

[Kedas]

You always have a rotation unless your force is going through the mass center of that free object. (but praktical that can't really be done there is always a mistake, only gravity can dot it perfect)

When there are other forces present that would generate the opposite rotation then the rotation will be zero.

Also keep in mind that in many situation the force isn't going through its mass center and still there is no rotation, that is because there are correcting forces present to put the object back in it's previous position.

for example when you see a roket launch on TV that is going wrong in many cases is't starting to change diretion fast (rotate) because something about the correcting forces failed.

 

But correcting forces don't have to be so advanced a simple arrow is also equiped with correction tools. at the end of the arrow there are feders these make sure that the arrow isn't starting to rotate during its flight.

if you put the feders before the mass center of the arrow (at the top) you will see that the arrow will rotate very easily.

[5614]

well there is of course the argument which says unless you are going in an EXACT straight line you will end up where you started and you would have gone in one massive circle.