In space, can you get a pencil spinning with a single touch?

Question

You're in space, zero gravity, and a pencil is floating in front of you.  If you tap the end of the pencil with your finger, my sense is that it should float away from you and also start spinning.

1.  Is this right?
2.  If this is right - I've learned that producing a moment on an object requires a force couple.  Your finger on the pencil is one force; where is the other one coming from?  (Do I need to think of the pencil as a chain of mass elements, or something?  Would the answer to 1. be different if the pencil were infinitely rigid?)

? · Accepted Answer

I'm pretty sure creating a moment does not require a couple force. It just requires that the net force is not applied directly at through the centroid of the object or in equilibrium negating each other. Creating only a moment without a resultant force or "translational" motion is when a force couple is necessary.

So, I think if you tap the end of a pencil in space and the force vector does not go through the center of mass of the object, it should rotate and translate.

Art Masterpiece · Answer

The pencil will spin around its center of mass, so there's your other force couple.
Also called CG or Center of Gravity.

In space, can you get a pencil spinning with a single touch?

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