In the Otto von Guericke problem, two horses pull in opposite direction on a sph
ID: 1271999 • Letter: I
Question
In the Otto von Guericke problem, two horses pull in opposite direction on a sphere that consists of two hemispheres that have been evacuated of air and are being held together by air pressure. The claim is that he may as well have just used one team of horses for one side of the sphere and just tied the other end up to a strong tree/wall. Is this because of Newton's third law? How is this?. I have read several explanations but they do not make sense to me. Relative to the sphere as a whole, the forces from the two teams of horses would cancel each other (ditto for the wall scenario). However, relative to the hemispheres, each hemisphere would feel itself being pulled from the other hemisphere, even if that pull was not enough to sever the bond. If one team was pulling the right side away by 1000N and the other team pulling the left side away by 1000N, would not the total force be 2000N? The internet is saying 1000N. Thanks! Jason
Explanation / Answer
Just assume the sphere as your system. When there are two sets of horses, the hemispheres are getting, say T newton force from each side. In case of the wall, the same force acts in the form of tension in the rope tied to the wall. It is Newton's third law.
Now, if you want to calculate the force, just use newton's second law of motion and be very careful while deciding the system. In case of the sphere, it is not acelerating, hence the net force on it must be zero.
In case of the hemisphere, the force on it is 1000N from the horses, but it is still not accelerating, hence the net force on it is also zero. In case of the sphere, this is an internal force but in case of the hemisphere, this is an external force.