Can you push down more than your weight?
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I never said you didn't go to the gym, I'm just giggling at the thought of you stood there explaining what your doing to the other people in the gym.Doc Al said:
"Now assuming that both the strings and pulleys are massless ..."
Field report from the gym
OK, so I did check out several machines at the gym at work. The gym is small, with only two machines that could function as a triceps pressdown machine (they do not have a machine dedicated to that function). Both were Cybex machines: A cable cross-over machine and a lat pulldown machines. So this is hardly representative of all machines.
Nonetheless, for the record, both machines use a simple pulley arrangement: no mechanical advantage. Both machines use a pin to select the number of plates you lift.
The plates on the crossover machine are just labeled by numbers in sequence, not anything that is obviously the weight. The numbers do not corresponding to 10 lb increments (as one might at first think); I checked by hanging an Ivanko 25-lb plate from the cable--it could easily raise the stack at position #4.
The lat pulldown machine looked obviously labeled in pounds: 12.5, 25, 37.5, etc. I hung that 25-lb plate from the cable--it was not enough to balance weight of the 25 lb stack. Only when I hung 27.5 lbs from the machine did it balance. So, at least for this particular setting, you'd have to exert more than 25 lbs of force to lift the 25 lb stack. (Which is quite reasonable.)
OK, so I did check out several machines at the gym at work. The gym is small, with only two machines that could function as a triceps pressdown machine (they do not have a machine dedicated to that function). Both were Cybex machines: A cable cross-over machine and a lat pulldown machines. So this is hardly representative of all machines.
Nonetheless, for the record, both machines use a simple pulley arrangement: no mechanical advantage. Both machines use a pin to select the number of plates you lift.
The plates on the crossover machine are just labeled by numbers in sequence, not anything that is obviously the weight. The numbers do not corresponding to 10 lb increments (as one might at first think); I checked by hanging an Ivanko 25-lb plate from the cable--it could easily raise the stack at position #4.
The lat pulldown machine looked obviously labeled in pounds: 12.5, 25, 37.5, etc. I hung that 25-lb plate from the cable--it was not enough to balance weight of the 25 lb stack. Only when I hung 27.5 lbs from the machine did it balance. So, at least for this particular setting, you'd have to exert more than 25 lbs of force to lift the 25 lb stack. (Which is quite reasonable.)
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Simple experiement. Try lifting your body so that all your weight is on the bar, maybe try standing on it if you can balance yourself with a nearby object. Compare your weight to the weights on the scale.
If the back of the seat is tilted forwards, you could exert upwards force on the back of the seat. You could also jerk your body downwards, but would be quickly repelled back upwards.
If the back of the seat is tilted forwards, you could exert upwards force on the back of the seat. You could also jerk your body downwards, but would be quickly repelled back upwards.