Rolling friction When an object rolls without sliding or tends to roll on the surface of another object, the rolling friction is caused by the deformation of the contact part of two objects under pressureHindrance, called "rolling friction".Its essence is static friction.
Rolling friction is generally measured by resistance moment, and the force is related to the nature of the object, the shape of the surface and the weight of the rolling object.Rolling friction is actually a kind of torque that hinders rolling.When an object rolls on a rough surface, if it is no longer subject to the action of power or dynamic moment, its motion will gradually slow down until it is still.In this process, rolling objects are subject to gravityelastic forceIn general, the contact part is subject toStatic frictionpower.Due to the deformation of the contact between the object and the plane, the object is trapped in the supporting surface under the action of gravity, and the object itself is also compressed and deformed. When the object rolls forward, the supporting surface in front of the contact rises, and the point of action of the resultant force N of the supporting surface acting on the object moves forward from the lowest point.It is this elastic force that produces a moment of resistance to the rolling of the object relative to the center of mass of the object, which is called rolling friction.For junior high school students, they have not yet mastered the concept of torque, so they should not talk about rolling friction as a kind of friction, but only about the impact of rolling on one object when rolling on another objectHindrance。Rolling frictionThe essence ofStatic friction。
Generally speaking, pressure makes objectsdeformationThe surface around the object is in a small slope shape, which is equivalent to climbing a small slope when rolling (there are small slopes on both sides).Gravity is generated when going up a small slopeGliding force, which is the resistance compared with a small ball.
Characteristics of rolling friction
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The rolling condition of an object is related to the contact surface. When a rolling object rolls or has a tendency to roll on the contact surface, both the object and the contact surface will occurdeformation。hisdeformationIt can be divided into contact surface deformation without deformation of rolling object (at this time, the object is calledrigid body）The contact surface does not deform (at this time, the contact surface is called the rigid surface) while the rolling object deforms, the contact surface and the rolling object do not deform, and the contact surface and the rolling object both deform.
The rolling object is a rigid body, and the contact surface is deformed
Rolling friction
The gravity G andSupportabilityNOn the same vertical line.When the rolling object rolls forward, it acts on the front bulge, the fulcrum moves forward to O ', and the force acting on the contact surface increases toN′。N'produces two effects: one is perpendicular to the direction of the contact surfaceN, which is related toGCounterbalance;The other is along the direction of the contact surfaceStatic frictionf，It prevents the ball from sliding forward, as shown in Figure 3.
Obviously, gravityGAnd supportNproduceCouple momentM resistance, its effect is to block the rotation of the object.Let the distance of O 'from the gravity action line beδ, then M resistance=Nδ。
If the vertical distance between the component f of N ′ in the direction of the contact surface and O is r, and O is the rotation axis, when the object rolls at a uniform speedMoment balance, then fr=N δ,.
The roller of the road roller can be simplified into this kind of problem when rolling the soil subgrade.
The contact surface is rigid, and the rolling object is deformed
When the contact surface is not deformed but the rolling object is deformed, the rolling of the object is essentially the overturning after deformation, but the fulcrum O 'moves forward.Similarly, gravityGAnd supportNOfCouple momentIs the resistance moment of the object rolling, and M resistance=Nδ，。
AutomobileBicycleThe problem can be simplified to this kind of problem when the tires of.
Both rolling objects and contact surfaces are rigid
If neither rolling object nor contact surface occursdeformationAt this time, the gravity and supporting force of the object are collinear balanced, that is, G=Nδ=0、r=0, rolling resistance moment M resistance=0, f=0, no rolling friction effect.
This is an ideal situation, but the movement of the train wheel on the rail can be simplified into this kind of problem.
Rolling objects and contact surfaces are deformed
The deformation of rolling objects and contact surfaces is the most common situation. Although this situation is very complex, it is between the first and second cases. According to the "two side clamping rule", the same conclusion can be obtained, that is, M resistance=Nδ、。However, when the shape of the rolling animal changes relatively large relative to the contact surface, it can be simplified as the first case;When the deformation of the contact surface is larger than that of the rolling object, it can be simplified to the second case.
About Rolling Friction
Why is it harder to ride a bicycle when it is flat?
You may have had such an experience whenBicycleIt's very difficult to ride when I'm not angry.What is the reason?This is where I started my little exploration.
To understand why it is difficult to ride when you are flat, you should first understandBicycleWorking principle of.When people pedal the car with their feet, the chain disk rotates, and the flywheel of the rear wheel driven by the chain rotates, so that the rear wheel also rotates.At this time, there is a backwardStatic frictionAs shown in Figure 3, the ground will also have a forward static friction force against the rear wheel.This force is the driving force of the car.Since the wheels roll on the ground, there must beRolling friction。However, the middle school has not conducted in-depth study on rolling friction, and the impact on rolling friction is not very clear, so it is necessary to check the information.
According to the data, when an object rolls on the surface of another object, it is subject to the resistance of the contact surface, which is called rolling friction.The condition of rolling friction is that the rolling object and the plane contact part must be deformed. The picture shows the deformation.
The feature of deformation is that the front of the rolling object forms a bulge, so the level of support forceComponent forceIt blocks translation, and the vertical component blocks rotation, so rolling friction occurs.
Then go backBicycleCome on, I already knowStatic frictionIt provides power, so the only reason why it is difficult to ride a bicycle without air is rolling friction.BicycleWhen the wheel is pressed on the groundElastic deformationThe ground will also deform, but the wheel deformation is the main one.WhenBicycleWhen there is no air, the deformation of the wheel increases, making the front bulge of the wheel more convexRolling frictionMoments make it difficult to ride a bicycle.
