# Playground swing paradox

1. Apr 25, 2005

### aviator

you are stopped sat in a swing and move your feet forward, the swing will acelerate backwards

you are rocking in the swing (forward) you move your feet forward and the swing acelerate forwards

how is that?

before you say you rise and lower your center of gravity adequately let me point that if this was the case there would be no diference between kicking in the same direction than the rocking of the swing or not but it makes a great diference

:surprised

2. Apr 25, 2005

### whozum

The brief moment your center of mass is not directly below the axis of rotation, you've exerted a torque on the entire swing system. When you restabilize your center of mass, this torque is still acting since gravity is providing the force. Once you stabilize your CM you will no longer exert a torque on the system aside from what you already exerted in the beginning. Each displacement of your CM from the direct vertical due to that first swing will also exert a smaller torque than the one before it.

Repeating this is what builds up the torque.

3. Apr 26, 2005

### aviator

i might be wrong but i think that when stopped if you move your feet forward ceratinly the swing moves backwards

but the center of gravity remains the same, you may vary the center of gravity vertically but not horizontally since the axe of the swing is the hold of vertical movement but it offers no resistance to horizontal movement, the efect in the horizontal axe while stopped is that of 0 gravity

in 0 gravity you cant expect to move your center of gravity by kicking with your legs

besides if by kicking forward you move your center of gravity backwards seems that while rocking in the swing is the oposite and you move the center of gravity forward as well

so when you kick forward where does the center of gravity move towards?

4. Apr 28, 2005

### aviator

"so when you kick forward where does the center of gravity move towards?"

if you dare to answer this question know you are an ignorant

if you say the center of gravity moves backwards you are an ignorant because then the swing would brake the rocking and it doesnt happen

if you say that it moves forwards you are an ignorant because that goes against newtons third principle

if you say the center of gravity doesnt move you are an ignorant because then how the hell does the swing rock