When the front wheel retreats, the driven wheel and the driving wheel of the rear wheel are driven by different forces. The thrust of the driven wheel acts on the axis of the wheel, and the force makes the wheel translate. If the ground is absolutely smooth and the wheels slide on the ground, the contact point with the ground tends to move forward relative to the ground. Therefore, if the ground is uneven, the friction force is backward, and the direction of the force does not pass through the shaft. Therefore, due to the driving gear structure, the shaft that pulls the wheel to rotate the driving wheel has the function of rotating the wheel when driving, and the contact point with the ground tends to move backward relative to the ground. If the ground is absolutely smooth, the wheels will slip in place, and if the ground is rough, the friction will move forward. Simply put, just like people walking and running with spikes, they "rake the ground" backwards, so the friction they receive is forward. It is best to prepare wheels when analyzing, and it is best to combine toy four-wheel drive with experiment! Explanation of Baidu search: Bicycles are similar to cars. When the rear wheels get power-bicycles use pedals, and cars rely on the power of the engine to make the rear wheels rotate forward. Their wheels are in contact with the ground and the direction of movement is backward relative to the ground. In this way, friction is generated on the ground to prevent the wheels from sliding backwards and moving forward. It is because of this friction that bicycles are good. The car can still go forward. If you put the rear wheel of the car high and let it leave the ground, no matter how hard you pedal, no matter how happy the rear wheel is, the car will not move half a step forward. Therefore, the forward friction of the rear wheel is essentially the driving force of the car. What about the front wheel? Imagine that the front wheel is braked, that is, it is not allowed to rotate, so that it will slide forward with the car body as it moves forward, so that the contact point between the front wheel and the ground tends to slide forward relative to the ground. In this case, the ground is facing the front wheel, which produces a backward friction force to prevent the wheel from sliding forward, and it is precisely because the front wheel is subjected to a backward friction force that the front wheel rotates. Furthermore, once the rear wheel is powered off, the situation of the rear wheel becomes the same as that of the front wheel, and the direction of friction immediately becomes backward, which should also be noted. When asked about the direction of friction between the front and rear wheels of a bicycle during movement, some students replied that the two wheels are in the same direction and both are forward. Because the front and rear wheels rotate in the same direction, the friction given to them by the ground is forward. This statement makes sense at first glance, but it is not the case in actual analysis. To understand this problem, we must first correctly understand the concept of friction. Friction is generated between two objects that are in contact with each other and have relative motion, which hinders the relative motion of the objects. In order to make the wheel move relative to the ground, the first thing to do is to act on the wheel, that is, the force that the foot acts on the pedal. It is because of this force that the rear wheel rotates counterclockwise. At this time, there is relative movement between the rear wheel and the ground, and friction is an obstacle to this movement. Therefore, the friction on the rear wheel is forward. So, what about the front wheel? It is not difficult to see that the front wheel tends to move forward because it is driven by the rear wheel. Because of this, the ground gives the front wheels backward friction. Therefore, the two wheels are rubbed in different directions. People call the rear wheel the driving wheel and the front wheel the driven wheel. When asked about the direction of friction between the front and rear wheels of a bicycle during movement, some students replied that the two wheels are in the same direction and both are forward. Because the front and rear wheels rotate in the same direction, the friction given to them by the ground is forward. This statement makes sense at first glance, but it is not the case in actual analysis. To understand this problem, we must first correctly understand the concept of friction. Friction is generated between two objects that are in contact with each other and have relative motion, which hinders the relative motion of the objects. In order to make the wheel move relative to the ground, the first thing to do is to act on the wheel, that is, the force that the foot acts on the pedal. It is because of this force that the rear wheel rotates counterclockwise. At this time, there is relative movement between the rear wheel and the ground, and friction is an obstacle to this movement. Therefore, the friction on the rear wheel is forward. So, what about the front wheel? It is not difficult to see that the front wheel tends to move forward because it is driven by the rear wheel. Because of this, the ground gives the front wheels backward friction. Therefore, the two wheels are rubbed in different directions. People call the rear wheel the driving wheel and the front wheel the driven wheel. When pushing a cart, the front and rear wheels rotate because of the friction exerted by the ground, that is, the front and rear wheels tend to move forward relative to the ground, so the friction between the ground and the front and rear wheels is backward. When riding a bicycle, people give the rear wheel a force through the chain to make it turn. Assuming that the ground is smooth, the rear wheel will accelerate forward, indicating that the rear wheel has a tendency to rotate backward relative to the ground, so the friction force facing the rear wheel is forward. The front wheel rotates due to the force. Assuming that the ground is smooth and the front wheel will not rotate, the friction force between the ground and the front wheel is backward. (When judging the direction of friction, it can be assumed that the contact surface is smooth and the moving direction of an object is its moving trend direction.

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