Is the net torque changed when a partner on a seesaw stands or hangs from her end instead of sitting? LxiddMmSWw unt V
Is the net torque changed when a partner on a seesaw stands or hangs from her end instead of sitting? (Does the weight or the lever arm change?)
3 Answers
No, if one simply hangs or stands, the force on the lever arm does not change. As, torque stands for $F \\times r$. Here, if the only affecting force is weight, then $F=mg$, (In case one tries to climb up the arm or jumps on it includes some extra forces.) and is directed straight down, as long as the lever arm is horizontal. If the person doesn't change his position, namely the $r$, the torque cannot change.
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2$\\begingroup$ The diagram does show a slight change in position, though -- the line of action of the girl's weight, through her COM, is slightly beyond the end of the see-saw plate, whereas the boy's weight acts slightly inwards from the end. $\\endgroup$ – Emilio Pisanty 22 hours ago
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$\\begingroup$ @EmilioPisanty Good point ! Besides girl can start to swing back and forward, thus making torque force oscillating from minimum to maximum in such way boy will have more fun :-D $\\endgroup$ – Agnius Vasiliauskas 22 hours ago
It does not matter how the weight is arranged, as long as it's center of mass (the midpoint of weight) stays the same distance from the pivot point. Moving farther from the pivot point increases leverage, moving closer decreases leverage. Of coarse putting your feet on the ground will lessen the weight on the see saw.
There is a slight difference due to the change in the lever arm between the hanging position indicated and a sitting position. the girl’s weight is applied to the very end of the see-saw plank; her center of mass will be directly below where she is holding the plank. When she’s sitting on the plank the effective lever arm will be shortened by something like 10cm. Your center of mass is, roughly, in the middle of your body behind your navel. If you consider where that point is for the boy in the figure you can see that it is a bit away from the very end of the plank.
