I would like to clarify whether an internal force in a system can cause it a motion externally? My Physics teacher and also other tuitions in my area tell that only external forces can cause motion of a system.
Last day I conducted an experiment (if you can call it one):
I filled a bucket half full of water then gave it a hard push (don't ask me the magnitude of force) on a smooth tiled floor. The observation was that after the bucket (didn't topple nor did the water spill out, it just reached the brim) skidded about a meter and a half, then it recoils about a foot (backwards). What I infer from this is that the inertia of the water (internal force) in the bucket (system) caused the water to exert adequate internal force on the (opposite) wall of the bucket which produced an external (recoil) motion for the system as a whole. I conclude that internal force can indeed cause external motion of the system (at times).
Now anyone who reads me, tell me if:
- I went wrong anywhere.
- my conclusion was wrong.
- my reasoning was incorrect.
Please be open and preferably detailed. I don't mind criticism, because I'm a learner. I'm humbly requesting your answers and opinions!
Best Answer
It depends what you mean by external motion.
If you had a box in space and you were sitting on one end with some baseballs and you threw a baseball at the opposite side then the whole box would move in the opposite direction as you threw the baseball, during the time the baseball was in flight.
Someone on the outside that can't see what you did would see the box move while the ball was in flight. If you had multiple balls you could move multiple times. But the momentum of the whole system of you, box, and baseballs isn't changing. And neither is the center of mass. The outside parts move because the internal parts are moving the opposite way.