Gym: How do I determine the dead weight (in Kg) of what I lift off a pulley system?

Question

For instance, the pec machine uses a pulley. How much do I truely lift?

For example, if I lift 18 plates and each plate weighs 10kg and 1 pulley is used, what is the dead weight? What is the formula?

Answer 1

It is often a problem in gyms with machines using pulleys. If the pulley is a single one, then whatever weight is on the stack is what you're lifting, but many machines have some sort of mechanical advantage, so there might be 100kg on the stack, but you aren't actually exerting 100kg force (kgf). On Earth a kilogram mass exerts a kilogram force downward, so we can ignore the "force" term.

I keep a little retractable tape in my gym bag so every time I come across a new machine, I do the following (it's easier if you have someone to help you):

Measure the distance the weights actually rise during the full range of the exercise, and multiply that by the total weight. That gives you total work in kg-metres. So you might have 50kg on stack, and your shoulder press raises it 0.45m. So that's 22.5kg-m of work.

Now measure the distance your hands move through the exercise (usually significantly further than than the weights). This might be 0.65m. Divide your work (22.5kg-m) by this distance (0.65m) to get the force at your hands, in this case 34.6kg. So if you did a free weight shoulder press, around 35kg would give you the same effort as 50kg on the machine.

A special case is the 45 degree sliding leg press machine. Your legs may push the slide about 0.5m, but the weights have only risen vertically by 0.5 x sine 45 deg, or 71% of the slide distance i..e. 0.35m. So pushing the 200kg on the rack is only 141 kg force at your feet, and your total work is 71kg-m. That doesn't count the platform weight of course.

I always record work as well as weight, as a heavy weight is not necessarily heavy work. As an extreme example, anyone can lift a wheel of a 1-ton truck off the ground with one hand - using a jack!

Answer 2

That depends on the machine. Count the number of lines (cables?) pulling on the weight. If you pull down on an overhead pulley attached to a 100 pound weight, only one line pulls up on the weight and tension in the line is 100 pounds. If you pull up on a pulley attached to a 100 pound weight and the other end of the line is attached overhead, there are two lines at the weight that share the load and you must pull up with only a 50 pound force. See picture in link.

Answer 3

weight=mass*9.8
mass is dead weight & weight is what pully measures