(1) Two students, A and B, conducted the experiment shown in Figure A, and both of them measured the deformation to determine the stiffness coefficient of the spring. According to Hooke's law k = mg △ x, there is k = mgd.

(2) Students use an object under the elastic force of the spring to convert the potential energy of the spring into

(3)A, according to the elastic potential energy of the spring, it is converted into gravitational potential energy.

Yes: 12kx2 = mgh.

k=mgd，

From the above solution, x = 2dh.

B, the elastic potential energy of the spring is converted into kinetic energy, and the kinetic energy is measured by the method of flat throwing motion.

Then the horizontal initial velocity can be obtained from the horizontal displacement and vertical height: v0 = L2Hg = LG2h,

So:12k (mgd) 2 =12m (lg2h) 2.

Solution: x = ld2h

However, both students made mistakes. A: It is difficult to accurately determine the maximum height of the ball. When the ball rises, it may contact with the inner wall of the plastic pipe, resulting in friction, and the experimental error is large.

B: During the experiment, the friction between the small iron ball and the desktop will bring errors to the experiment.

So the answer is: (1) Determine the stiffness coefficient k of the spring.