Both couple (moment) and force can make an object roll, but they can translate the instantaneous center. Friction in the rolling process is the reaction force caused by this translation action. Like the friction with sliding tendency, it is also static friction. The magnitude of this static friction (F) has a great influence on the rolling of objects. Previous discussions have been conducted on the assumption that F is greater than power (P or Pm). When F is less than P (or Pm), objects will slide or rotate, which is very common in our daily life. For example, if the feet are slippery (that is, the friction is small), people will have a hard time walking; Cars will "skid" when driving on icy roads. They all show that friction has a great influence on rolling motion. That is to say, for rolling, friction is a necessary condition to stabilize the instantaneous center so that it does not shift. The greater the friction, the stronger the instantaneous center stability; No friction, no rolling.
Description: deformation affects the actual contact state between objects, enhances mechanical meshing and molecular bonding, thus improving friction; Moreover, with the occurrence of motion, the transformation of the deformed part of the object, that is, dynamic deformation, will also consume some energy and show resistance; Some objects with large deformation will also produce obvious form and position resistance. For example, trains need to lay tracks; Skis should have a certain length and the front end should be tilted upward to overcome the influence of large deformation. In daily life, we often come into contact with friction. For example, the bottle and brush in your hand will not slip, which is the result of static friction; There are also many applications in production technology. For example, a belt conveyor conveys goods through the static friction between the goods and the conveyor belt. There are two common physical phenomena:
1。 Men are walking.
People's walking is equivalent to the rolling of a polygon, and half the stride is equivalent to the arm of force. According to the direction of friction, it can be judged that human walking is rolling caused by rotation (torque) effect. Obviously, when people push, the direction of friction is backward, which is caused by translation effect.
2。 The movement of bicycles
The rear wheel of a bicycle rolls by the rotating torque transmitted by the chain, which is also called traction. The direction of the force at the contact point with the ground is backward (opposite to the rolling direction), so the direction of static friction is forward; The front wheel is rolling caused by the translation force transmitted by the front axle, and the direction of the force at the contact point with the ground is forward. So the direction of static friction is backward.