Question

getting up from chair!

Answer 1

I just want a confirmation of whether i have missed something or not!!

Answer 2

Ok :))

Answer 3

as long as N2 is to the LEFT of my cm
its impossible to get up.. (unless i use my hands )

cause N1 cannot change..
thus to get up.. we need to bring our feet all the way to the RIGHT of the CM and then push up.. so now N2 can help me create a torque!

Answer 4

is that correct? :D

Answer 5

i tried it.. its amazing!!

Answer 6

but i wanna see if i missed out something..

Answer 7

I think it's ok.
When you bend forward, the aim is to align your feet and CM on the same vertical line. So that you can push while keeping an e

If your CM is to the left, you will get up and rotate forward, which can happen if you start running from your seated position.

If your CM is still to the right and you push with your feet, you will rise, but fall backwards onto your chair.

Answer 8

yup yup.. thats exactly what i was thinking too.. such a beautiful concept .. a demo that can always be done in class to blow their minds away!!!

Answer 9

also.. i thought of including hands.. even when i use hands.. i need to put it to the Right of the CM.. else if i put it to the LEFT of CM, again its useless.. right?

Answer 10

Simple experiment: Face a wall so that your toes are touching the wall. Try to stand on tiptoes. It is impossible, because your CM cannot be in the vertical plane of the wall.

Answer 11

wait lemme try

Answer 12

haha.. correct.. there will always be a torque.. due to the normal force onto my CM :D

Answer 13

do you have more on this?.. or any link..

Answer 14

"also.. i thought of including hands.. even when i use hands.. i need to put it to the Right of the CM.. else if i put it to the LEFT of CM, again its useless.. right?"

Right, you will also rock forward.

Answer 15

"haha.. correct.. there will always be a torque.. due to the normal force onto my CM :D"

?? there is no normal force on your CM. Your weight will create the torque that takes you backwards.

Answer 16

i mean't normal force of my toes.. wll create a torque about my cm

Answer 17

"i mean't normal force of my toes.. wll create a torque about my cm"

Ok, both points of view are the same, but as your toes are the axis of rotation, it is simpler to speak of torque created by your weight about this axis.

Do you know this one when you are crouching on your knees (like in a muslim prayer), arms backwards, and you must tilt a match box with your nose?
I will try to send it to you.

Answer 18

lol.. i think i have heard of that.. .i think i get what u mean.. lemme try!

Answer 19

Well in order to prove the point, the position of the match box must be at arm's length of your knees. I'll try to find a picture.

Answer 20

wait.. i can't tilt the matchbox.. at ALL.. regardless of what position i am in? :P

Answer 21

how do u tilt a matchbox with your nose? :D :D!!

Answer 22

If the matchbox is too far away, you fall. If it is close enough, you succeed. But....

Answer 23

hey wait.. before that.. in my original question i got a doubt..

the pivot of rotation in the chair problem is my feet right?.. so if i consider moment about that point then

my weight makes me sit, and my butt normal force N1 makes me get up..

isn't it possible to make sure that torque due to N1 is more than weight? theoretically?

Answer 24

well? cause since N1 is farther away from my foot ? can't it create higher torque?

Answer 25

so if i think about it that way, it has nothing to do with N2 and my foot.. :(

Answer 26

"isn't it possible to make sure that torque due to N1 is more than weight? theoretically? "

No, it can't, since at the beginning net force = 0

Answer 27

oh i get i i get it..

its like.. since net TORQUE about my feet is zero..

Answer 28

initially..