Today, we introduce two kinds of homemade toys that achieve a special downhill effect by changing the center of gravity and fulcrum. There seems to be nothing special to talk about in principle, so let's talk about how to make them respectively. (in order to see the motion clearly, the dynamic pictures in the article all decelerate slightly)

this gadget can roll downhill on a slope with a rough surface, and a glass bead is hidden inside the cylindrical paper tube. The rolling of glass beads in the paper tube drives the change of the center of gravity and the roll of the paper tube. Place the surface to have enough friction, too smooth paper tube may directly slide down, the effect of laying towels is better.

first make a paper tube, in fact, basically find a piece of printing paper and cut it into a piece of paper. The length of the paper tube I made is about 4.5 cm, and its diameter is slightly larger than that of a glass ball. You can roll the glass ball with a piece of paper and roll it into a tube, then relax and paste it. As long as the glass ball can roll without hindrance in the paper tube.

then cut two cross-shaped pieces of paper (as shown below). There is no strict size requirement, these two pieces of paper are used to block both ends of the cylinder and the glass ball inside the cylinder.

(first stick the cross paper on one side, put it into the glass ball and then glue the other side)

it is said that it is OK to put it into the ball with an empty capsule shell, but I do not have a ball of the right size, so you can try it if you are interested.

② walk downhill

this is a walking downhill kangaroo, consisting of a body with forearms and legs that can do limited swings.

drawings are as follows (source: http://scigame.ntcu.edu.tw/power/power-049.html)

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I used 0.5cm thick airplane wood (a light plank that is often used to make aircraft models). You can print and cut the paper and stick it on the board and cut it according to the outline. The plane wood is actually very easy to cut. I think it can be done with an art knife, but the saw is a little difficult to control. Cut off the rough outline of the board and then polish the edges with sandpaper or file.

the method of assembly is not complicated, that is, the two legs are placed on both sides of the body of the kangaroo and worn with bamboo sticks. The diameter of the hole at C is larger than the rotating shaft of the bamboo stick, so that the legs can rotate flexibly, while at A, as long as the bamboo stick can get through. Place B can pass through a wire, which further ensures that the rotation of the left and right legs can be synchronized. At the same time, the wire will stop when it touches the bulge of the abdomen, limiting the rotation range of the leg.

not too close to the middle body, you can separate them with a gasket:

this kangaroo needs more debugging than rolling downhill. As for slopes, apart from having a certain degree of friction, we should also pay attention to the fact that the slope is not too large, otherwise it is easy to overturn. The kangaroo may lean to one side or dump in the downhill. At this time, check whether the rotating shaft is horizontal and the height of the two feet is the same, and you can adjust it again. Thicker boards should be easier to keep stable (0.5cm is actually a lower limit, but not stable), and kangaroos should not be too large (the head and tail length of kangaroos is not more than 12cm on the Science Game Lab website). In short, compared with the first one, it takes some patience to play with it.