Does Raising My Arm Change the Center of Mass?

[Physics guy]

Let me imagine myself standing on the Earth with my arm in the resting position perpendicular to the ground. Now if I decide to raise my right arm by 90 degrees, now that it is parallel to the ground. I have shifted my center of mass in this process. But the center of mass will not accelerate unless there is an external force on the system. I can only think of internal force that is acting, in this case is the force exerted by my muscles. Is my understanding flawed somewhere?

 

[PeroK]

Summary:: Acceleration of center of mass when there is no external force

Let me imagine myself standing on the Earth with my arm in the resting position perpendicular to the ground. Now if I decide to raise my right arm by 90 degrees, now that it is parallel to the ground. I have shifted my center of mass in this process. But the center of mass will not accelerate unless there is an external force on the system. I can only think of internal force that is acting, in this case is the force exerted by my muscles. Is my understanding flawed somewhere?

There is an external force on your feet from the Earth. That's how you are able to walk.

 

[Physics guy]

There is an external force on your feet from the Earth. That's how you are able to walk.

Well this isn't a question of walking or the normal force from the ground. Okay let us suppose that instead of in Earth I am in deep space and I lift my arm. Then in this case what is the external force which shift my center of mass.

 

[A.T.]

Let me imagine myself standing on the Earth with my arm in the resting position perpendicular to the ground. Now if I decide to raise my right arm by 90 degrees, now that it is parallel to the ground. I have shifted my center of mass in this process. But the center of mass will not accelerate unless there is an external force on the system. I can only think of internal force that is acting, in this case is the force exerted by my muscles. Is my understanding flawed somewhere?

Try to do it on a skate board, while holding some weight in your hand (for better effect).

 

[A.T.]

Well this isn't a question of walking or the normal force from the ground.

The feet also transmit frictional forces.

Okay let us suppose that instead of in Earth I am in deep space and I lift my arm. Then in this case what is the external force which shift my center of mass.

Your center of mass will not shift in this case.

 

[Physics guy]

Try to do it on a skate board, while holding some weight in your hand (for better effect).

So does that mean that I will shift to the left when I raise my right arm.

 

[PeroK]

Well this isn't a question of walking or the normal force from the ground. Okay let us suppose that instead of in Earth I am in deep space and I lift my arm. Then in this case what is the external force which shift my center of mass.

If you are in deep space and you move your arm, then the rest of your body will move in the opposite direction. Newton's third law.

 

[A.T.]

So does that mean that I will shift to the left when I raise my right arm.

Yes, the rest of your body will also change orientation a bit.

 

[hmmm27]

In deep space, with no external influence, the CoM stays in the same place. You might be able to move your body around it, but it remains fixed.