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I recently quit the local gym (budget restrictions) and started to workout from home instead.

I bought a little set of 2x30kg adjustable dumbbells, and built a little bench for doing dumbbell bench press. This being quite a light weight, I bought some 10kg plates to add up on those dumbbells.

My big surprise was that it felt incredibly more difficult to press, say only 2x30kg with the following setup:

  • 5|10 === 10|5 x2

than with this one:

  • 2|2|2|4|5 === 5|4|2|2|2 x2

The 10kg plates I bought are indeed quite large, but I didn't expect this to have an impact on the workout difficulty. Can anyone help me understand what's happening?

Thanks!

So Jacques
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    Did you try and weigh the plates individually? My question is are the plates really weighing the same '?kg' they meant to be? – Freakyuser Aug 20 '13 at 04:05
  • Wow, I admit I didn't even question this point, and assumed everything was right. Will check later today. Thanks for the insight. – So Jacques Aug 20 '13 at 05:03
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    You say that they feel more difficult to press, but are you sure that is because they feel heavier? Or do they feel more difficult to control? –  Aug 20 '13 at 06:21
  • If I had to guess, I'd say the larger plates are forcing you to hold the dumbbells further away from your body so that you can get the same stretch in your chest before pressing. That's going to change how much vertical force you can generate, possibly require more stabilisation, etc. – Anthony Grist Aug 20 '13 at 22:03
  • While interesting, this question appears to be off-topic because it is about physics. The answer to this question will not help anyone's fitness. –  Aug 21 '13 at 15:25
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    I disagree, this is on topic as it discusses different configurations of equipment. – Baarn Aug 21 '13 at 16:11
  • @Informaficker So, as long as one mention fitness equipment, we can ask physics questions about them? –  Aug 25 '13 at 18:34
  • @Kate if it has an impact on the persons exercise, I think there is nothing wrong with it. There will always be crossovers to other areas of expertise within fitness. (And just because there is physics.SE doesn't mean that we can't have overlapping topics) – Baarn Aug 25 '13 at 19:34
  • @Informaficker I doubt that the reason for a particular dumbbell setup feeling heavy impacts a person's fitness. –  Aug 25 '13 at 21:16
  • i found the same thing..The more lighter weights on the bar as opposed to a few heavier ones (that had the same total weight value) made it easier to bench press...Ive told folks this over the years and they all say its all in my head..BS...I want answers..A friend suggested it was the spreading that helped bend the bar downwards on either end which helped me lift it with more ease...But thats crap too since the bar never bent – Clay Sep 20 '17 at 19:46

1 Answers1

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The narrower your center of gravity the harder it is to balance the plate. Think about trying to press a relatively light bar that was 30 feet across. It would be very taxing to balance this bar even if it didn't weigh much. The balancing will happen from the use of your muscles, and the wider the weight distribution makes it seems heavier - due to more muscle interaction for the same "weight". Physics of lifting.

DMoore
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  • This is very true, more plates means the weight is spread out wider (more difficult to balance). – RealityDysfunction Aug 20 '13 at 13:58
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    What does it mean for a center of gravity to be narrow? The center of gravity is a single point. –  Aug 20 '13 at 14:56
  • Well maybe I should say pivot point but center of gravity works too. See your center of gravity is the space between your hands on the bar. Since you have two access points (given that your hands are centered) the entire space should represent the center of gravity. With an olympic bar most of the weight is very close to the center of gravity or pivot point (I don't like that term because we have two equalized points). The fact is the further away the weight is from the center not only is there greater balancing needed but their is diagonal force put on the axis points. – DMoore Aug 20 '13 at 17:23
  • This is basic physics. Go in the gym with an olympic bar and do a lift that you have to push and that you can do a fair amount of weight - say 45s on each side. Now take the 45s off and do 9x5s on each side. Trust me the 5s will feel much heavier and you will not be able to do near the amount of reps/weight. – DMoore Aug 20 '13 at 17:24
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    Your answer seems to totally contradict what the person asking the question has observed (they're saying fewer heavier plates feels more difficult than more lighter ones). Also, what you're saying about centre of mass is totally wrong, assuming the plates actually weigh exactly what they're supposed to. – Anthony Grist Aug 20 '13 at 18:10
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    @DMoore, this seems logical to me, issue is, in the setup I describe, the version with the more plates feels lighter than the one with fewer, larger plates. – So Jacques Aug 20 '13 at 18:47
  • Well what I described isn't a "feeling" it is the science of a lever system. So you are back to your weights are calibrated to their "labeled weight" correctly. – DMoore Aug 20 '13 at 19:05
  • @DMoore Basic physics says that center of gravity is a single point, not "the space between your hands on the bar". –  Aug 20 '13 at 21:21
  • Center of gravity is indeed a single point, and it should always be in the middle of the handle, since everything is highly symmetric. What @DMoore describes sounds more like he is talking about "Moment of Inertia" http://en.wikipedia.org/wiki/Moment_of_inertia or "Angular Momentum" http://en.wikipedia.org/wiki/Angular_momentum – Jens Schauder Aug 21 '13 at 11:55
  • center of gravity is not fixed when there are two pivot points as there is when there are two hands on a bar. Often in layman science this is referred to as center of mass. Sorry but the terminology is right, maybe too simplified. As you introduce additional pivot points on an object its center of mass or bidirectional influences change. – DMoore Aug 21 '13 at 14:24
  • As long as the bar doesn't change, the center of gravity and center of mass is fixed. Layman science never calls having two pivot points on a bar the center of gravity or center of mass. Adding pivot points does not change an object's center of mass or center of gravity. –  Aug 21 '13 at 15:23
  • In this case you have to divide the bar into sections, each side having its own center of mass. The further these points are apart the harder it is to balance the weight. However as the question asks for something different, I think the answer is much simpler (check first comment below question). – Baarn Aug 25 '13 at 19:46
  • @Informaficker Why do you need to divide the bar into sections? I can arbitrarily choose sections that cause their centers of mass to be close together or far apart. –  Aug 25 '13 at 21:17