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I feel like this question has been asked in so many ways, but I am still unable to explain my personal experience based on previous answers. From other discussions:

  • You can always replace an off-center force by the same force centered plus a torque r×F.
  • Trajectory and acceleration of the COM are the same you would get with the same force centered.

But when I apply a perpendicular (no component going through COM), off-center force to an object that I am holding in one hand by its pivoted COM, I feel no force/ acceleration at the pivot. If I push through the pivot point, I feel the force/ acceleration immediately.

Per the above, shouldn’t the 2 scenarios impact the pivot point/ COM in the same way. Does having a pivot point change the situation? Hard to understand since I am not feeling any reaction at the pivot point that might cause things to change.

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  • $\begingroup$ When you apply the force off center are you allowing the object to rotate (spin)? $\endgroup$
    – Bob D
    Apr 5, 2023 at 21:02
  • $\begingroup$ Physics can explain your experience if you give us the details on how you apply a single force and how you measure the results. Human perceptions can be easily fooled so you need some objective way of performing this experiment. $\endgroup$ Apr 6, 2023 at 11:46
  • $\begingroup$ youtube.com/watch?v=vWVZ6APXM4w $\endgroup$ Apr 6, 2023 at 11:47
  • $\begingroup$ and follow up facebook.com/veritasium/videos/… $\endgroup$ Apr 6, 2023 at 11:48
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    $\begingroup$ There are two standard ways to measure a force. One is to apply a counter-force to cancel it. One is to measure an effect such as acceleration or angular acceleration and then calculate the force needed to produce this effect. You are probably using your hand to apply a counter-force. If the force and counter-force are not aligned, then they cannot cancel. The measurement will not be accurate. The further shifted one is from the other, the less each will effectively resist the other. $\endgroup$ Apr 6, 2023 at 21:36

2 Answers 2

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Does having a pivot point change the situation?

If I understand you correctly, you're probably not feeling the same force because you are allowing the object to rotate about the pivot point where you are holding it.

If you prevent it from rotating (by grasping firmly) you will feel the same force when you apply the force to the pivot point as when you apply it offset from the pivot point. The difference is when you apply the force offset you will feel, in addition, a twisting effect on your your hand due to the torque.

Hope this helps.

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But when I apply a perpendicular (no component going through COM), off-center force to an object...

Physics problems often assume some sort of constant force, but it's really hard to apply such.

For an object that rotates freely, you will probably find it difficult to apply a significant force on the object. It's similar to trying to push a feather in the air with a force of 10N. It can be done, but probably not by any human.

Your force on the spinning object is simply very small, so it's difficult for your other hand to detect.

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  • $\begingroup$ This answer was very useful. It is a little frustrating as I seem to be able to generate significant angular momentum and energy and not really feel any reaction at the pivot. I suppose I need something with a larger moment of inertia but still able to spin freely. $\endgroup$
    – JohnC
    Apr 7, 2023 at 18:25

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