# Lifting objects from their center of mass =zero torque?

If i lift an object like a dumbbell from its center of mass, torque would be zero,however we don't generally lift a barbell from its center of mass(we lift them near the sides) ,the dumbbell should be much easier to stabilize since there will be no torque when we lift them from their center of mass. However many researches show that you can lift more weight with a barbell than a dumbbell(about 20% more), they claim that lifting a barbell will require less activation of stabilizing muscles, isnt the torque zero when lifting a dumbbell therefore it should be easier to lift the dumbbell than the barbell? If not Shouldn't the torque be less when using a dumbbell since you lift it from or near its center of mass?

P.S:also as discussed here(https://fitness.stackexchange.com/questions/25941/are-barbells-really-more-beneficial-than-dumbbells), you may be able to lift about 25% more weight while using a barbell than the weight you can lift using a dumbbell.

Torque is never 0 when lifting anything with the human body due to the fact that your joints only allow for pivotal motion. Torque is also relative to where the lifting appendage is connecting to your body. Given that none of our appendages can lift as a car jack would (complete vertical lift with no horizontal component), torque will always be a factor. To answer your question, no, the torque is not 0 when lifting a dumbbell. Further, torque is only lessened with a barbell due to a variance in technique compared to a one armed curl with a dumbbell.

• Shouldn't the torque be less when using a dumbbell since you lift it from or near its center of mass? Nov 5, 2019 at 21:45
• You are in contact with the dumbbell at ITS center of mass, yes. BUT you are not lifting with your hand! You are lifting with your shoulder and elbow using power provided by a large portion of muscles on whichever side of the body you are lifting. Your hand is just what is attaching your arm to the dumbbell, but the actual force to move your arm comes from closer to your body. So you aren't actually applying the force to the dumbbell's center of mass, you are applying it to your arm which in turn applies it to the dumbbell. You see? Nov 8, 2019 at 14:50
• but your arm also applies the force to the dumbbell,the barbell has its center of mass in the middle,however we lift them from the sides,but we lift the dumbbell with out arms from the center,why do I need more stabilizing muscles to hold the barbell? Nov 8, 2019 at 15:12
• No, only your hand applies the force to the dumbell. Your hand is the only thing in contact with it. It is easy to lift a dumbell with one hand because it is so short and you can easily control the angular momentum with the force of a single hand. However, a barbell is much longer. By gripping with two hands equidistant from the center, you are effectively lifting the bar from its center of mass, while gaining a form of leverage on it's inertia. Nov 12, 2019 at 20:22
• The barbell center of mass is in the center, "By gripping with two hands equidistant from the center, you are effectively lifting the bar from its center of mass"in my opinion you aren't lifting the barbell from its center of mass because you are not gripping the center of the barbell, am i right? Nov 12, 2019 at 20:45

You are confusing the force your hand pushes on the dumbbell versus the force that you are really applying. It is true that the normal force between your hand pushing on the dumbbell and the dumbbell is not providing any torque. But as CuriousOne mentioned, your body can only lift the dumbbell with pivotal motion. It is the muscles in your bicep that actually lift the dumbbell, and they do it by essentially pulling on your forearm.

The above image of lifting a dumbbell should help. Imagine the forearm was a door and the elbow joint being the pivot where its attached to the wall. Obviously pulling on the door with $$F_M$$ provides a torque.

• What about the barbell, researches show that it is easier to lift a barbell, however it should be easier to lift the dumbbell than the barbell since there will be less torque,am i right? Nov 6, 2019 at 7:41
• The torque is roughly the same. All you do is add an extra arm, resulting in an additional $F_M$ force (and more weight on the bar). Its true your hands arent acting on the center of mass of the barbell, but again that doesn't matter because the important force in lifting the object is the tension in your biceps. Nov 6, 2019 at 15:32
• i have heard it is because you activate more stabilizing muscles when lifting a dumbbell, we lift dumbbells from their center of mass, however we don't lift a barbell from its center of mass, howcome that the dumbbell needs more force to balance it? Nov 6, 2019 at 18:37
• Howcome the torque is roughly the same? Aren't we lifting the dumbbell from its center and the barbell from its sides? Nov 6, 2019 at 18:39