(1) The dotting frequency of the dotting timer is 50hz, and the dotting period is 0.02s. When the heavy objects begin to fall, the falling height of the heavy objects in the first dotting period is h=

1

2

gt2≈

1

2

×9.8×(0.02)2m≈2mm，

Therefore, the distance between the first two points of the selected paper tape is close to 2mm, and the distances between the first point and the second point on each paper tape are 0. 12cm, 0. 19cm and 0.25cm respectively.

It can be seen that there is a big error in the operation of classmate C. The specific reason is that the paper tape is loosened first and then the power supply is turned on. When a point is fixed, the speed is already there, resulting in a distance of more than 2 mm between the first two points. But classmate A's operation was wrong, because the paper tape was greatly resisted in the falling process.

(2) According to the fact that the speed at the midpoint of uniform linear motion is equal to the average speed in the process, there are:

vb=

0.0706? 0.03 14

2×0.02

Every second = 0.98m/s.

(3) According to the functional relationship, when the dot timer hits point B, the reduction of gravity potential energy of the weight is:

△EP = mgh = 1.0×9.8×0.05j = 0.49j

△ek=

1

2

mvb2=0.48j

(4) Through calculation, the value is △ EP > △ ek. This is because there is frictional resistance in the process of falling objects.

(5) The conclusion of the experiment is that the mechanical energy of falling body is conserved within the error range.

So the answers are: (1) A and C; When the paper tape falls, there is great resistance. Loosen the paper tape first, and then connect the power supply.

(2)0.98

(3)0.49; 0.48

(4)>; There is frictional resistance

(5) The mechanical energy of falling body is conserved within the error range.