How many forces does the eraser absorb on the magnetic blackboard?

How many forces does the eraser absorb on the magnetic blackboard?

Four!
The force analysis of it: first, it is subjected to gravity, vertically downward; secondly, it is subjected to the magnetic force given by the blackboard, and the vertical plate faces inward; and then look at its state: static! So it should also be subjected to the supporting force given by the blackboard, and the vertical plate faces outward; since it has the supporting force, then it has to be subjected to the friction force given by the blackboard, and vertically upward

Why can the words on the blackboard be easily erased by the blackboard eraser

Because the attraction between the molecules that make up the blackboard and the molecules that make up the chalk is far less than the attraction between the molecules that make up the blackboard, the words written are easy to erase and the blackboard will not be erased

The teacher rubs the chalk on the blackboard with the blackboard, which belongs to. A point moves to form a line, B line moves to form a plane, C plane moves to form a body, and D is wrong

B