"Balance of Common Point Forces" Force and Balance PPT Courseware

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"Balance of Common Point Forces" Force and Balance PPT Courseware

The first part of the content: [Learning literacy·clear goals]

Physical concepts: 1. Know the concept of equilibrium state and balance of forces. 2. Understand the concept of equilibrium of objects under the action of common point forces. 3. Know the equilibrium conditions of common point forces.

Scientific thinking: 1. Be able to use the equilibrium conditions of common point forces to answer practical problems. 2. Master common methods of dealing with equilibrium problems.

Balance of Common Point Force PPT, Part 2: Independent Preview and Exploration of New Knowledge

Knowledge sorting

1. Equilibrium conditions of objects under the action of common point forces

1. Equilibrium state: The state in which an object ______ or does ______.

2. The equilibrium condition of an object under the action of a common point force: the resultant force on the object is ____, that is, Fsum = 0.

3. balance of forces

(1) Balance of forces: The resultant force of several forces acting on an object is ______. This situation is called the balance of forces.

(2) Two-force balance condition: The two common-point forces acting on the object have magnitude ______, direction ______, and act on the same straight line.

(3) Three-force balance condition: When three common-point forces are balanced, the ______ of any two forces must be equal in magnitude and opposite in direction to ______, and act on the same straight line.

2. Application of equilibrium conditions

1. Static equilibrium of an object: An object is in a stationary state under the action of a force, which is called static equilibrium. The ______ of the external force on an object in static equilibrium is zero.

2. The balance of an object in a certain direction: When the ______ experienced by a moving object in a certain direction is zero, the object is in a state of equilibrium in that direction.

Basic self-test

1. Think and judge (mark “√” if correct, “×” if incorrect)

(1) If an object is in equilibrium, then the object must be in a stationary state. ( )

(2) When the speed of an object is equal to zero, the object must be in equilibrium. ( )

(3) An aircraft flying at a constant speed in a straight line at high altitude is in equilibrium. ( )

(4) During the 100-meter race, the athletes are in a state of balance at the moment of starting. ( )

(5) An object whose combined external force remains constant is in equilibrium. ( )

2. Five forces at a common point are balanced, then which of the following statements is incorrect ()

A. The resultant force of four of them is equal to and opposite to the fifth force.

B． The resultant of three of the forces is in equal and opposite direction to the resultant of the remaining two forces.

C. The sum of the five forces is zero

D． If three of the forces are removed, the object must be unbalanced

3. As shown in the figure, the block moves to the right and in the horizontal direction at a constant speed under the action of force F. The resultant direction of the friction force f and the pulling force F on the block should be ()

A. Horizontally to the right B. straight up

C. Upward to the right D. Upward to the left

The balance of common forces PPT, the third part: cooperative exploration to overcome difficult problems

balance of forces

1. Understand the equilibrium state from the perspective of kinematics: a balanced object is at rest or in a state of uniform linear motion. In this state, its acceleration is zero, that is, the acceleration of an object in equilibrium is zero; conversely, an object with zero acceleration must be in a state of equilibrium.

2. Understand the equilibrium state from a dynamic perspective: the net external force on an object in equilibrium is zero, and conversely the net external force on an object is zero, so it must be in equilibrium.

3. Static balance and dynamic balance

(1) Static equilibrium is equilibrium in a stationary state, and the resultant force is zero.

(2) Dynamic equilibrium is the equilibrium state of uniform linear motion, and the resultant force is zero.

4. Equilibrium state and balance of forces

[Example 1] The object is acted upon by a common point force, then the correct one of the following statements is ()

A. An object whose velocity is equal to zero at a certain moment must be in equilibrium

B． An object that remains at rest relative to another object must be in equilibrium

C. If the net force on an object is zero, it must be in equilibrium

D． An object must be in equilibrium when it moves with uniform acceleration

Thoughts: Whether an object is in equilibrium must be judged based on the equilibrium condition (that is, whether the resultant force on the object is zero).

regular method

The difference between "still" and "v=0"

(1) The object remains at rest: It means v=0, a=0, the resultant external force on the object is zero, and the object is in equilibrium.

(2) There are two possibilities for the object’s movement speed v=0:

①v=0, a≠0, the resultant external force on the object is not equal to zero, the object does not remain stationary and is in a non-equilibrium state, such as the object thrown to the highest point.

②v=0, a=0, this situation is consistent with the static state in (1).

For training

1. Regarding the equilibrium state, which of the following statements is correct ()

A. An object in free fall is in equilibrium at its highest point

B． The wooden block is placed on the inclined surface of the inclined plane and moves to the right with the inclined plane at a constant speed. The wooden block is in a state of equilibrium.

C. The wooden block is placed on the inclined surface of the inclined plane and accelerates uniformly to the right together with the inclined plane. The wooden block is in a state of equilibrium.

D． A cup resting on a horizontal table in a uniformly accelerating train is in equilibrium.

B [ When an object in free fall motion is at its highest point, its speed is zero, but the resultant force is not zero, and it is not in equilibrium. A is wrong; the block and the inclined plane are relatively stationary. If the whole body moves in a straight line at a uniform speed , then the wooden block is in a state of equilibrium. If the whole block moves in a straight line with uniform acceleration, the block also has acceleration and is not in equilibrium. B is correct, C is wrong; the train, table and cup move with uniform acceleration as a whole, and the cup also has acceleration. Not in equilibrium, D is wrong. ]

Equilibrium conditions and applications of common point forces

1. The balance condition of common point force: If the resultant force of common point force is zero, then the resultant force in two mutually perpendicular directions must also be zero, that is, Fx sum = 0, Fy sum = 0.

2. Four common corollaries of equilibrium conditions

(1) Two force balance conditions: the two common point forces are equal in magnitude and opposite in direction.

(2) Three force balance conditions: When three common-point forces are balanced, the resultant force of any two of them must be equal to the third force, opposite in direction, and on the same straight line.

(3) When an object is in equilibrium under the simultaneous action of n common point forces, the resultant force of these forces in any direction is zero. The resultant force of any (n-1) force must be equivalent and opposite to the nth force, acting on the same straight line.

(4) When an object is in equilibrium under the action of multiple common-point forces, the forces connected end to end must form a closed polygon.

3. Two notes on dealing with balance issues

(1) When an object is balanced by three forces, it is relatively simple to use the force decomposition or synthesis method.

(2) When solving a balance problem and establishing a coordinate system, as many forces as possible should coincide with the coordinate axes, and as few forces as possible need to be decomposed. When an object is acted upon by more than four forces, the orthogonal decomposition method is generally used.

regular method

How to solve the common point force balance problem

(1) Synthetic method: When an object is balanced by three common point forces, the resultant force of any two forces must be equal in magnitude and opposite in direction to the third force.

(2) Decomposition method: The object is balanced by three common point forces. If a certain force is decomposed according to its effect, then its component force and the other two forces meet the equilibrium conditions.

(3) Orthogonal decomposition method: When an object is acted upon by three or more forces, the forces on the object are decomposed into two groups that are perpendicular to each other, and each group of forces satisfies the equilibrium condition.

(4) Triangular method of force: For an object that is balanced by three forces, translate the vector diagram of the force so that the three forces form a vector triangle connected end to end, and solve the problem based on mathematical knowledge such as the sine theorem, the cosine theorem or similar triangles. Unknown force.

Balance of Common Point Force PPT, Part 4: Reaching Standards and Solidifying Double Bases in the Hall

1. (Multiple choice) The following statement about the balance and equilibrium conditions of common point forces is correct ()

A. If the speed of an object is zero, it must be in equilibrium

B． If the speed of an object remains constant, it must be in equilibrium

C. If an object is in equilibrium, the net force on the object in any direction must be zero

D． If an object is in equilibrium under the action of three common-point forces, then the resultant force of any two forces is equal in magnitude and opposite in direction to the third force.

CD [ When the moving speed of an object is zero, it is not necessarily in a state of equilibrium. Error A. When the speed of an object remains unchanged and the direction changes, it is not moving in a straight line at a uniform speed. It must not be in a state of equilibrium. Error B; The object is in a state of equilibrium. When , the resultant force is zero, the resultant force of the object in any direction must be zero, C is correct; the resultant force of any two common point forces is equal to the third force in the opposite direction, the resultant force is zero, the object is in equilibrium, D is correct. ]

2. An object is placed on an incline, as shown in the figure. When the inclination angle of the incline gradually increases and the object is still stationary on the incline, the object is subjected to ()

A. The combined force of gravity and supporting force gradually increases

B． The combined force of gravity and static friction gradually increases

C. The combined force of supporting force and static friction gradually increases

D． The combined force of gravity, supporting force and static friction gradually increases

A When the object is stationary, the object is affected by three forces: gravity mg, support force N and friction force f. From the orthogonal decomposition, we know that N = mgcos θ, f = mgsin θ, and θ is the inclination angle of the inclined plane. The resultant force of the three forces is zero, the resultant force of gravity and support force is in the same direction as the static friction force, the inclination angle becomes larger, the static friction force becomes larger, A is right. The resultant force of gravity and static friction is in the opposite direction to the supporting force. The inclination angle becomes larger and the supporting force becomes smaller. B is wrong. The resultant force of supporting force and static friction is in the opposite direction of gravity and remains unchanged. C is wrong. The resultant force of gravity, supporting force and static friction is always zero, D is wrong. Therefore, choose A.]

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Update Time: 2024-09-29

This template belongs to Physics courseware Lu Ke Edition High School Physics Compulsory Course One industry PPT template

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