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  • if you lift a large box alone, you try to lift it from the Center(Center of mass),but if the box is small you just use both of your hands each grabbing a side of the box(not from center of mass).
  • if two person try to lift a table they each grab a side of the table and lift it(not from center of mass),but if you try to lift the same table alone you try to lift it from it's center(center of mass).
  • if you try to lift a dumbbell you use one hand to lift it from it's center(center of mass),but if you try to lift a barbell you lift it far away(from the sides) from center(center of mass).

why don't we always lift stuff from it's center(center of mass) since it can help maintain balance of the object?

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4 Answers 4

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why don't we always lift stuff from it's center(center of mass) since it can help maintain balance of the object?

Just an opinion, but I think it is because if we attempt to focus our force on the center of mass (COM) with one hand, but are off the COM, we need to apply a torque as well to counteract the moment of the center of mass about the point where we are applying our force, in order to keep the object stable (keep from tipping over). If we are using one hand, we would need to twist our wrist to obtain that torque. Not always easy to do.

On the other hand (no pun intended) by using both hands on either side of the COM if the COM is not exactly in between, we can balance the moments about the COM by applying more upward force on one of the hands. Applying an upward force, as opposed to twisting is an easier way make the necessary torque adjustment.

ps.

I used to lift weights. Have you ever tried to lift a barbell with one hand at the center? If you are a little off the COM the barbell tends to twist your wrist when you try to keep it stable. Than can cause an injury.

Hope this helps.

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  • $\begingroup$ If you lift an object from its center the toque will be zero, because gravity will do its job, right? $\endgroup$
    – Zheer
    Oct 4, 2019 at 16:32
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    $\begingroup$ @Zheer only if the COM is exactly in the middle of the palm of your hand when you grasp the bar. Ca be difficult to do if you have ever tried to raise a barbell with one hand. It will twist your wrist if you are a little off $\endgroup$
    – Bob D
    Oct 4, 2019 at 17:14
  • $\begingroup$ So why don't we lift the barbell from its center using both hands? No torque? $\endgroup$
    – Zheer
    Oct 4, 2019 at 18:29
  • $\begingroup$ @Zheer I used the example of one hand because that was the example you gave in the third bullet. Certainly two hands in the middle are better than one hand, but not a as stable as spreading the hands apart. For one thing in order to keep the concentrated masses on each end level, you will again need to twist your wrists instead of just adjusting the upward force on your separated hands which is a heck of a lot easier. You rarely see weight lifters lift the bar with two hands in the middle because of that. $\endgroup$
    – Bob D
    Oct 4, 2019 at 19:01
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    $\begingroup$ @Zheer Let me see if I can add some diagrams to my answer to help visualize things. $\endgroup$
    – Bob D
    Oct 4, 2019 at 22:20
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We almost always lift the object from its center of mass. When there are two people lifting an object or when we lift using both our hands, we do so from the opposite ends so that the net force is along the center of mass. If we don't do so, there will be an extra torque produced which will make it much harder to lift. However if the object is really small (a pen),we can lift it from a single end because the extra torque is very small and is insignificant to us.

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  • $\begingroup$ So there will always be extra torque even if you lift the object from its center like the pen,right? $\endgroup$
    – Zheer
    Oct 4, 2019 at 16:15
  • $\begingroup$ No, there wont be any torque if the object is lifted from the center of mass. You need to apply force away from the center of mass for there to be any torque. $\endgroup$
    – Sam
    Oct 4, 2019 at 16:49
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I don't know what do you mean by "lifting from the center of mass" since the center of mass of, say, an empty homogeneous cubic box, is the geometrical center of it and there is just "air" there, no "box".

Some notes on this anyway: if you don't want the object to rotate when lifting it, the torque on the object must be zero. There are different ways to achieve this, mainly placing equal forces on opposite ends. Choosing differnt opposite ends will give you different stability on the lift.

Without getting more technical (in this case look for principal axis of inertia), lifting the object by the longest horizontal opposite ends will give you the best stability. If you lift something by yourself, it may be that you don't reach those ends and that's why you lift it differently as if more people lift the same object.

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  • $\begingroup$ If you lift the object from center, torque will be zero, because gravity will pull the object from both ends, right? So why do we need to lift the object from it's sides? $\endgroup$
    – Zheer
    Oct 4, 2019 at 16:27
  • $\begingroup$ You can do that too (think about attaching a rope in the middle of a box with a handle and lifting it). However the box will most likely rotate, at least in the horizontal plane, since nothing is holding it in that direction and small deviances from equilibrium will get it moving. $\endgroup$ Oct 4, 2019 at 17:11
  • $\begingroup$ Let's assume that two person is lifting a big table, why don't they just lift it from the center of the object? Is it because ettiquette or something similiar? $\endgroup$
    – Zheer
    Oct 4, 2019 at 18:33
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An object will rotate if ALL net forces combined do not center on the center of mass. Say you pick up a plate by the edge, with one hand, you will likely have some fingers on the bottom pushing up, and your thumb on top, farther outward toward the edge, pushing down. This will equalize the two forces at the center.

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