"Application of Newton's Laws of Motion" Newton's Laws of Motion PPT

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"Application of Newton's Laws of Motion" Newton's Laws of Motion PPT

Part One Content: High Frequency Test Point 1 Calculation of Instantaneous Acceleration

[Knowledge Comprehensive]

Newton's second law is the instantaneous action law of force. Acceleration and force are generated, changed, and disappeared at the same time. When solving for instantaneous acceleration, attention should be paid to the following two physical models:

Rigid rope model (thin steel wire, thin wire, etc.)

The occurrence and change process of deformation lasts very short time. At the moment when the force condition of the object changes (such as a certain force disappears), its deformation can suddenly change.

Light spring model (light spring, rubber rope, elastic rope, etc.)

The deformation is obvious, and it takes a long time for the deformation to change. In an instantaneous problem, the size of the elastic force can be regarded as constant.

[Training Unicom]

[Typical Example 1] As shown in Figures A and B, none of the thin lines in the figure can be extended, and the two small balls are in equilibrium and have the same mass. If two horizontal thin lines are suddenly cut off, the acceleration of ball A at the moment of cutting is __________ and the direction is ________; the acceleration of ball B is ________ and the direction is __________; the inclined thin line OA in Figure A The ratio to the tension of the spring in Figure B is ________ (the angle θ and the acceleration of gravity g are known).

[ Analysis At the moment of the thin line, the acceleration a1 direction of the small ball A is downward along the tangent direction of the circumference, that is, the vertically inclined thin line OA is downward. FT1=mgcos θ, F1=mgsin θ=ma1

So a1=gsin θ

[Regular method]

Ideas for solving the instantaneous problem of Newton's second law

(1) Determine whether the given problem is a rope type or a spring type, and determine whether the elastic force can undergo a sudden change.

(2) To analyze the force on an object, the "look ahead and look back" method can be adopted, which not only analyzes the force before the motion state changes, but also analyzes the force at the moment when the motion state changes.

[Instant training]

1. As shown in the figure, three blocks A, B, and C of equal mass are connected by light springs between A and the ceiling, and between B and C. A and B are connected by thin ropes. When the system is at rest and the string between A and B is suddenly cut, the accelerations of A, B, and C at this moment are (positive in the orientation) ()

A． -g, 2g, 0B. -2g, 2g, 0

C. 0, 2g, 0 D.-2g, g, g

Analysis: Before cutting the string, perform a force analysis on B and C as a whole. They are balanced by the total gravity of B and C and the pulling force of the string, so T = 2 mg. Then analyze the force on block A and find that it is balanced by the gravity and the tension of the string. The tension of the rope and the tension of the spring can be seen from the force balance of block A. The tension of the upper spring F1 = 3mg. From the force analysis of block C, it can be seen that the tension of the lower spring F2 = mg. After cutting the string, the gravity and the elastic force of the spring remain unchanged, and the pulling force of the string is reduced to zero. Therefore, the resultant force on block B is equal to 2mg. When going downward, the resultant force on block A is 2mg. When going upward, the resultant force on block C is 2mg. The force remains unchanged and the resultant force is zero, so block B has a downward acceleration with a magnitude of 2g, block A has an upward acceleration with a magnitude of 2g, and block C has an acceleration of zero, so choose B.

PPT on the application of Newton's laws of motion, Part 2: High-frequency test point 2 Two types of basic problems in dynamics

[Knowledge Comprehensive]

1. Two basic problems in dynamics

(1) The force on the object is known, and the motion of the object is studied. That is, when the force on the object is known, the acceleration of the object is found, and the motion of the object is determined based on the kinematic formula.

(2) The movement of the object is known, and the force on the object is studied. That is, when the movement of the object is known, the acceleration of the object is obtained by the kinematic formula, and then the force on the object is determined from the acceleration.

2. How to apply Newton's second law to solve problems

Conduct force analysis and motion state analysis on the research object, clarify the physical situation and initial state described in the question, and then use "acceleration" as a bridge connecting force and motion according to Newton's second law, combined with kinematic formulas Solve.

[Training Unicom]

[Typical Example 2]As shown in the figure, a wooden block with mass m slides upward along an inclined plane with an inclination angle θ at a certain initial velocity. The inclined plane is stationary and long enough. The kinetic friction between the wooden block and the inclined plane The factor is μ.

(1) Find the magnitude and direction of the acceleration when the wooden block slides upward.

(2) If the wooden block can slide down the incline after reaching its maximum height, find the magnitude and direction of the acceleration when the wooden block slides down.

[Regular method]

Two tips for applying Newton’s second law

(1) If an object is accelerated by only two forces, the resultant force is generally calculated according to the parallelogram rule, and the direction of the resultant force is the direction of acceleration.

(2) If an object is accelerated by more than two forces, the orthogonal decomposition method is often used to solve it. The principle of establishing a coordinate system is to have as many vectors as possible fall on the coordinate axes. Therefore, depending on the situation, either force can be decomposed, or acceleration can be decomposed. When decomposing the force, the acceleration is often made on a certain coordinate axis and the resultant force on the other coordinate axis is zero.

[Instant training]

4. In order to reduce injuries to passengers caused by sudden braking during driving, people designed seat belts. Assume that the passenger mass is 70 kg, the car speed is 90 km/h, and the time required from applying the brakes to a complete stop is 5 s. The force exerted by the seat belt on the passenger is approximately (excluding the friction between the person and the seat) ()

A． 450N B. 400N

C. 350N D.300N

5. As shown in the figure, a sufficiently long horizontal conveyor belt runs at a constant speed at v0 = 2 m/s. At time t = 0, a small slider of mass m is gently placed on the left end. The kinetic friction between the slider and the conveyor belt is known Factor μ=0.1, acceleration of gravity g=10 m/s2, then the speed of the slider at t=1.5 s is ()

A． 3 m/s B. 2.5m/s

C. 2 m/s D.1.5 m/s

PPT on the application of Newton's laws of motion, part 3: High-frequency test point 3: Image problems in dynamics

[Knowledge Comprehensive]

1. common image forms

In dynamics and kinematics problems, common and commonly used images are displacement images (st images), velocity images (vt images) and force images (Ft images), etc. These images reflect the movement patterns and force of objects. Laws, rather than representing the movement trajectories of objects.

2. Image analysis methods

When encountering a problem with a physical image, you must carefully analyze the image. First understand the information given by the image from its physical meaning, points, line segments, slopes, intercepts, intersections, inflection points, areas, etc., and then use the common point force Problem solving on equilibrium, Newton's laws of motion and kinematics formulas.

[Training Unicom]

[Typical Example 3] As shown in Figure A, a long wooden board with mass M is placed at rest on a rough horizontal ground. There is a block with mass m that can be regarded as a particle. It moves with a certain horizontal initial velocity. Rush up the plank from the left end. In the process from when the block rushes onto the board to when both the block and the board are stationary, the vt images of the block and the board are shown as the polylines in Figure B. According to the vt image (g is 10 m/s2), find:

(1) The dynamic friction factor μ1 between m and M and the dynamic friction factor μ2 between M and the ground.

(2)The ratio of m to the mass of M.

(3) In the process from when the block hits the board to when both the block and the board come to rest, the displacements of the block m and the long board M relative to the ground.

[Regular method]

Techniques for handling image problems in dynamics

vt image

The direction of movement, instantaneous speed, displacement and acceleration within a certain period of time can be obtained from the provided image, and the force on the object can be determined based on the actual movement situation.

Ft image

First of all, it should be clear whether the image represents which force the object is subjected to, or the resultant force, and the acceleration is determined based on the force on the object to study its movement.

atimage

Pay attention to the positive and negative acceleration, correctly analyze the motion of each segment, and then formulate equations according to Newton's second law based on the force on the object.

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Update Time: 2024-07-06

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