# How can I move a chair with no external forces acting on it?

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If I sit on a chair with my legs off the ground and I suddenly jerk my body forward, I move the chair forward. Why am I able to do this? Static friction might be a factor but if both the chair and I are one system how did I start moving in the first place? Are there any other factors at play besides static friction? any input is appreciated, thanks.

In: Physics

You are moving the centre of mass in one direction by jerking your body and that movement overcomes the friction and moves the chair.

You and the chair may be part of the same system, but you’re generating energy within the system by swinging your legs (chemical energy gets converted to mechanical energy). This energy has to go somewhere, and it’s enough to overcome static friction by a small amount.

Sudden movement of your centre of gravity in one direction and then holding the chair overcomes the static friction of the chair on the floor. Slowly moving your weight back keeps within the limit so the chair stays still. Repeat the cycle and you move across the room. It wouldn’t work as well on ice or a chair with castors.

Because you push off the chair which is solid on the ground then use your greater inertia to put the chair with you. The ground provides the counter force to your aftions

It’s really just static friction and rolling resistance, like you say. Without those your center of mass would stay in one place. Ask yourself if the same thing would happen flotaing in space, without the floor. No, so there are external forces acting on the system of you plus the chair.