Urgently ask for a junior high school physics problem, solve a junior high school physics problem

The best way to do this is to draw an image to solve it, when the object goes from infinity to two times the focal length, like from one time to two times the focal length, and it takes the same time, so the speed of the natural object is greater than the speed of the image.

When an object goes from two times the focal length to one time of the focal length, it is like walking from two times the focal length to infinity, and the time taken is the same, so naturally the speed of the image is greater than the speed of the object.

When an object walks from a double focal length to the center of light, the direction of the image first changes instantaneously, and then it takes the same time to walk from infinity to the center of light, so the velocity of the image is naturally greater than the speed of the object.

This is a bad explanation!

Mostly junior high school! The derivatives of high school can be explained, but for you they are fantastic.

So the only way to do this is to make a diagram of the standard points.

Objects move equidistantly, but elephants move not equidistantly.

As for why, it is the convex lens that changes the path of light, which is equivalent to expanding or shrinking!

It's like the relationship between a map and an actual distance!

The classical formula of geometric optics is: 1 object distance + 1 image distance = 1 lens focal length.

The focal length does not change, so the image distance = focal length * object distance (object distance - focal length), this is obviously not a linear relationship. The object distance is x, the image distance is y, and the focal length is 1.

1, first is the camera principle, the image distance is reduced, then the slide projector principle, the image distance is magnified, 2, so it is b

Since there is no diagram, then I can only roughly say the solution.

Look at the height of the liquid level of the glass tube in the middle of the glass bottle.

Holes higher than the liquid level in the glass tube will not allow water to flow out.

Holes that are lower than the liquid level in the glass tube will flow water.

This is because the air pressure at the liquid level in the glass tube is equal to the water pressure here, and in the area above it, the water pressure is lower than the air pressure, so the air is drawn in, and the water pressure is higher than the air pressure in the lower area, and the water shoots out.

Suppose the top cube has a mass of m1, an area of each face is s1, and a volume of v1

The second cube has a mass of m2, an area of each face is s2, and a volume of v2

The k-th cube has a mass of mk, an area of each face is sk, and a volume of vk

The twentieth cube has a mass of m20, an area of each face is s20, and a volume of v20

then s2=4s1, s3=9s1,......sk=(k^2)*s1……s20=400s1

v2=8v1,v3=27v1,……vk=(k^3)*v1……v20=8000v1

Because the pressure on any contact surface is equal, (m1+m2)*g s2=m1*g s1

So m1 + m2 = 4m1, m2 = (4-1) m1 = 3m1

In the same way, m3 = (9-4) = 5m1

mk=[k^2-(k-1)^2]*m1=(2k-1)*m1

m20=(20*20-19*19)m1=39m1

m1/v1=ρ

m2/v2=(3m1)/(8v1)=(3/8)ρ

mk/vk=[(2k-1)m1]/(k^3*v1)=[(2k-1)/k^3]*ρ

m10/v10=(19m1)/(1000v1)=(19/1000)ρ

m20/v20=(39m1)/(8000v1)=(39/8000)ρ

of which 20=(39 8000) is the smallest.