What causes unhinged objects to rotate?

[etotheipi]

I can't understand how couple can be formed, doesn't all internal forces cancel out and only external force acts.

You are correct, it is the external force which determines the angular acceleration as well as the translational acceleration of the configuration, all internal forces cancel pairwise.

In addition to this, if you work in an accelerating frame of reference (for instance the centre of mass frame), you must also include external fictitious forces in your analysis (specifically, assuming the frame is non-rotating, a fictitious body force $-m\vec{a}$ that acts through the centre of mass). Since the centre of mass has zero acceleration in the centre of mass frame, we can determine that an additional fictitious force $-\vec{F}$ will act through the centre of mass in this accelerating frame. But this is not a reaction force, it is a different beast entirely!

 

[jbriggs444]

Now only question left is why object rotates when force is applied?

I thought I'd posted that already in #26. It is a requirement based on torque and angular momentum conservation.

To say it with pompous verbosity...

Given the constraints on the motion of the myriad bits of pancake-stuff imposed by the rigidity of the pancake, the motion of the pancake can be completely characterized by a linear translation rate of the center of mass and a rotation rate of the bits as they orbit that center in lock-step.

Given the magnitude, direction and application point of an external impulse on a pancake that is initially at rest, there is exactly one possible linear motion and exactly one possible rotation rate that can result.

We have calculated said rates and determined that an on-the-center force results in no rotation.

[For simplicity, we have restricted our attention to two dimensions and rotation in the plane]

 

[sophiecentaur]

Sir I can't understand how couple can be formed, doesn't all internal forces cancel out and only external force acts.

Do you really doubt that the object will spin? I'd recommend starting with the fact that it does and then try to explain it by finding whatever flaw there is in your understanding of the situation.
Can you really believe that only one of the balls will move or that the other one will magically move in a straight line parallel to the struck ball? Try it on a flat surface with two masses joined with string.

 

[sophiecentaur]

To say it with pompous verbosity...

I like your style, young man.

 

[sophiecentaur]

You need to do some serious work to clarify what you are talking about here. As written it is false, false, false.

Hah, I fear you may be right there. And with no verbosity, either.

 

[Frigus]

I thought I'd posted that already in #26. It is a requirement based on torque and angular momentum conservation.

I thought I'd posted that already in #26. It is a requirement based on torque and angular momentum conservation.

To say it with pompous verbosity...

Given the constraints on the motion of the myriad bits of pancake-stuff imposed by the rigidity of the pancake, the motion of the pancake can be completely characterized by a linear translation rate of the center of mass and a rotation rate of the bits as they orbit that center in lock-step.

Given the magnitude, direction and application point of an external impulse on a pancake that is initially at rest, there is exactly one possible linear motion and exactly one possible rotation rate that can result.

We have calculated said rates and determined that an on-the-center force results in no rotation.

[For simplicity, we have restricted our attention to two dimensions and rotation in the plane]

Thanks,
I got it!
If we take granted that angular momentum is always conserved then it makes a lot of sense.