Purpose and requirements

Knowing the function of axle can save labor, and the amount of labor depends on the ratio of wheel radius to axle radius.

Instruments and equipment

Axle model (J2 120 model), dynamometer (J2 104 model), iron frame, hook code, string and meter ruler.

As shown in figure 1.46- 1, the model of the axle is a secondary bakelite cylinder installed on the axle. Diameter of large cylinder 120mm, and diameter of small cylinder 60mm. There is a small hole for the tether in the groove of each cylinder. The bearing plate on the bracket is made detachable, which is convenient for loading and unloading the cylinder. The bearing plate is equipped with a friction brake. A strut is installed at the bottom of the bracket for fixing the axle. A wire handle is installed in a hole at one end of the shaft, one end of the shaft is bent into a right-angle hook, and a small hole is drilled at the hook end. The other end of the handle is equipped with a metal cylinder to adjust the balance, and the handle can be fixed by shaft end screws after moving.

experimental method

1. As shown in figure 1.46-2, fix the axle model on the iron frame, and adjust the arm length of the handle (that is, the distance from the right angle hook of the electric wire to the center of the axle) so that the ratio of the handle length to the radius of the large and small cylinders is 3: 1 and 6: 1 respectively.

2. Loosen the friction brake on the axle, and then tie one end of the string to the side slot hole of the small cylinder of the axle model, and hook the code at the other end. Turn the handle so that the rope is wound around the cylinder.

3. Hang the dynamometer on the small hole at the end of the handle, and pull the handle through the dynamometer by hand to balance the axle. Write down the reading of the dynamometer, then pull the dynamometer to rotate the cylinder to other positions, and observe the reading of the dynamometer when the axle is in different positions (always keep the pulling direction perpendicular to the handle). It can be found that the readings of the dynamometer are all equal, which is equal to 1/6 of the code weight hooked on the rope when the experimental error can be ignored. By changing the weight of the hook on the rope and repeating the experiment, the same conclusion can still be obtained: the radius of the wheel is 6 times of the radius of the shaft, and the force acting on the wheel is equal to 1/6 of the force acting on the shaft.

4. Tie the rope to the hole in the side groove of the big cylinder, and repeat the above experiment to get a similar conclusion.

5. Change the arm length of the handle and repeat the experiments in 3 and 4 above.

Finally, it can be concluded that the radius of the wheel is several times that of the shaft, and the power acting on the wheel is a fraction of the resistance acting on the shaft.

6. This experiment can also be demonstrated with hook code without dynamometer. Take off the handle and use the big cylinder as the wheel and the small cylinder as the shaft. A thin rope is wound around the wheel and the shaft in opposite directions, and the other end of the rope is hooked to adjust the weight of the hooked yard. It can be found that when the axle is balanced, the ratio of code weight hooked by ropes at both ends of large and small cylinders must be 1: 2.

Matters needing attention

1. In the experiment 1-5, when the hand pulls the handle through the dynamometer to balance the axle, the pulling direction of the dynamometer must always be perpendicular to the handle.

2.J2 120 axle model is an instrument specially designed for middle schools to demonstrate axle functions. When selecting, it should be noted that the step wheel rotates flexibly relative to the shaft and the shaft rotates flexibly relative to the bracket, and the end face of the shaft should not jump obviously when rotating.

3. The structure of axle model should be clearly explained to students.

reference data

1. Demonstrate the moving distance between the point of force and the point of resistance when using the axle. In the above experimental device, the meter scale can be fixed on the iron frame so that the meter scale is parallel to the upright post of the iron frame. In the experiment, the initial position of the hook code is marked on the meter scale, and the handle is pulled by the dynamometer to rotate at a uniform speed for one circle, and the distance traveled by the end of the handle is calculated, and the rising distance of the hook code on the rope is recorded, which shows that using the wheel shaft can save labor (this is the main purpose of using the wheel shaft), but the point of force needs to move more distance.

2. homemade axle. Taking a wooden stick with a diameter of 2 cm and a length of about 20 cm as the shaft and a wooden disc with a diameter of 15 cm as the wheel, a round hole with a radius slightly smaller than the shaft is dug in the wheel, the shaft is inserted into the hole, and a thick iron nail is nailed at the center of the shafts at both ends as the rotating shaft and put into the supporting hole of the bracket; It is made into an axle. Tower wheels can also be glued with round wooden boards with a thickness of about 1cm and diameters of 20cm, 15cm, 10cm and 5cm respectively. In order to improve the visibility, each CD is marked with clearly distinguishable colors. The wheel radius and shaft radius of this kind of axle can be selected at will, which is very convenient to demonstrate. Due to the large mass of the shaft, bearings should be installed to reduce friction.

Editor's Note: This small experiment can be supplemented by the physics experiment teaching of "mechanics" to cultivate and improve students' experimental ability and literacy.