Workout physics

Have you ever been astonished when your friend, who trains at that other gym, tells you that he manages to make the same amount of reps on cablecross as you do, but with the double amount of weight? Then read this!

Today when I was at the gym, I overheard two guys talking about two cable machines. One of them claimed that the weight was heavier on one of the machines, even though they had the same numbers and markings. The other guy said something like “Well, you can clearly see that the weight blocks on this machine is bigger”.

I checked it out and found out that: The weights were exactly identical. It had not only the same markings, but the size of the blocks were also exatly the same size (with the same height, width and length dimensions, so it was easy to see). Then how come that the weight was so much heavier on one of the machines?

The answer lies in how the weights are attached to the cable.

The red arrow “T” is the force that you pull with. This gives a tension in the rope, with the same size of the one that you pulled with. Since it’s a rope, the tension will be equally big in all of the rope. This means, if the cable is attached to the weight with a pulley, the weight will be affected by twice the force of the one that you pulled with. If it’s without a pulley, you will simply lift the whole weight “without any help”.

How much easier does it get with a pulley then? Well, since the double of your force is acting on the weights, you will need half as much force to drag it up. The weight will feel as if it was only half the weight. Pretty easy, don’t you think?

Comment: This effect is something that I have thought about some time on the gym now. If it says on the machine that I put on 20 kgs, but I only get to apply the pressure of 10 kgs, isn’t that quite decepting? I believe so. It’s sometimes hard to know how strong you actually are! Also, something else that I have been thinking about, while at the subject; why is it that the weights don’t say if they are in pounds or kgs? Sometimes they do, but most of the time they don’t. If it only says 40, then maybe I’ll go “Aha they mean that it’s in pounds, so that’s about 20 kg’s. This can’t possibly be 40 kgs”. But what if the weight has a pulley? Then the weight could be 40 kgs, but still feel as 20 kgs.

Ah, this got longer then I expected. Hope you learned anything at least. Stay safe at the gym!