Image appearance when u ＜ F, u ＞ 2F, u = f, f ＜ u ＜ 2F, u = 2F

Image appearance when u ＜ F, u ＞ 2F, u = f, f ＜ u ＜ 2F, u = 2F

U2f is an inverted and reduced real image with different sides, and the image is between one and two times the focal length
U = f no imaging
f

If u is equal to F, if u is equal to 2F, if f is less than u, if f is less than 2F,

Is gravity acting on all objects on the earth?
If there is no mass, is there gravity?

Yes, for the time being, there is no so-called object that is not subject to gravity. The force on the object due to the attraction of the earth is called gravity. The direction is vertical and downward. The magnitude of gravity on the object at the same point on the ground is directly proportional to the mass m of the object, expressed by the formula g = mg. Usually near the surface of the earth, the value of G is 9.8 N / 1000

Are objects near the earth's surface subject to gravity

It is not accurate to say that an object is subject to gravity. If it is in the center of the earth, what it is subject to is the sum of all parts of the earth's gravitation. But these forces are balanced, so it will be reflected as a state of no force. In other words, the object will "lose weight" in the center of the earth, just like in a spaceship