The torque changes the rotating objectmotion state Ofphysical quantity, doors, windows and other rotating objects fromQuiescent stateWhen changing into a rotating state or changing from a rotating state to a static state, it must be subjected to force.However, if we apply the force on the rotation axis of the door and window, no matter how much force is applied, the motion state of the rotating object will not change. It can be seen that the motion state and change of the rotating object are not only related to the size of the force, but also related to the direction of the forceAction point of forceIs related to.The farther the action point of the force is away from the axis of rotation, the more perpendicular the direction of the force is to the plane of the axis of rotation, and the more obvious the force changes the motion state of the rotating object.In physics, the influence of the action point and direction of the force on the change of the moving state of the rotating object is comprehensively expressed by the physical quantity of torque. Therefore, torque is defined as force andLever armThe product of.Torque summarizes all the laws that affect the movement state of rotating objects. Torque is a physical quantity that changes the movement state of rotating objects.
）: the vertical distance from the rotation axis to the action line of the force;
Calculation formula of torque and angular momentum
(2) Moment（
or
）：
, the unit is N * m;
(3) Moment describes the rotating effect of force on objects;
(4) Torque is a vector. In middle school, only clockwise and counterclockwise directions are considered.Generally, the counterclockwise torque is positive and the clockwise torque is negative.
Equilibrium condition
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(1) The balance of an object with a fixed axis of rotation means that the object is stationary or rotates at a constant speed around the axis of rotation;
(2) FixedRotating shaftThe balance condition of an object is that the resultant moment is zero, that is
, that is, the sum of clockwise torques is equal to the sum of counterclockwise torques.
General equilibrium conditions:
The resultant force is zero, and the resultant moment is zero, that is
，
。
Torque judgment
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When listing the moment equation, determine which forces are generatedClockwiseMoment, which forces produceanti-clockwiseMoments are important.The action point of force can be compared withRotating shaftConnect, decompose the force along the connecting line direction and the vertical connecting line direction, and then judge the direction of the moment generated by this force.
Key points analysis
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1．Rotating balance: an object with a rotating shaft is at rest or rotating at a constant speed under the action of force.
It is important to define the shaft:
In most cases, the rotation axis of an object is easy to determine, but in some cases, you need to determine the position of the rotation axis yourself.For example, if a long wooden stick is placed on the horizontal ground, its two endpoints are
, now
Add one at the endverticalThe upward external force makes the rod just leave the ground, and calculate the force
Size of.In this question
The horizontal line of the point perpendicular to the rod is the rotation axis of the rod.Like this, before solving the problem, it is necessary to determine many problems of the shaft through analysis. Only when the shaft is clear, can the torque be calculated, and then the torque can be usedEquilibrium condition。
The effect of a force on the rotation of an object is not only related to the size of the force, but also related to the arm of force, that is, the rotation effect of the force on the object is determined by the torque. ①When the arm is equal to zero, regardless ofForceHow big it is, it will not have a rotational effect on objects. ②When the force is parallel to the rotation axis, it will not rotate the object. The key to calculate the torque is the force finding arm.The attention arm is the distance from the rotation axis to the action line of the force, not the distance from the rotation axis to the action point of the force.
(2) The force with a certain size isMaximum torqueConditions for:
① The force acts on the point farthest from the rotation axis;
② The direction of the force is perpendicular to the forceAction pointConnection with the shaft.
(3) Calculation of moment:
① Calculate the force arm first, and then calculate the moment by definition
If the arm of force F is
moment
② First, decompose the force in two directions parallel to and perpendicular to the rodComponent forceThere is no rotation effect on the rod, and the torque is zero;The moment of the component force perpendicular to the rod is the product of the magnitude of the component force and the length of the rod.
Such as force
The moment of F is equal to its component force
Generated torque,
The two methods are different, but the results are the same. Choosing an appropriate method for a specific problem will simplify the process of solving the problem.
3．Moment balance condition:
The algebraic sum of moments is zero or the sum of all the moments that make the object rotate clockwise is equal to the sum of all the moments that make the object rotate counterclockwise,
Make necessary discussions and write clear answers.
5．Correctly understand the concept of torque
Moment is a vector. In middle school physics, the forces acting on the object are all in the same plane, and the moment of each force on the shaft can only make the object rotate clockwise or counterclockwise. In this way, the resultant force of several moments is simplified asAlgebraic operation。
