"Average Speed ​​and Instantaneous Speed" Simple Movement of Matter PPT Courseware

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"Average Speed ​​and Instantaneous Speed" Simple Movement of Matter PPT Courseware

[Experience] Observe changes in the speed of object movement

Activity 1: Observe a car driving smoothly on a straight road.

Activity 2: Let the ball fall still from a certain point as shown in the picture.

(1) In activity 1, the speed of the car ______ changes.

(2) In Activity 2, the speed of the ball is getting more and more ______ (choose "big" or "small").

Tips: (1) None (2) Big

pre-study questions

1.What is average speed?

2.What is instantaneous speed?

3. How to calculate average speed?

Explore average speed and instantaneous speed

Combining "experience" and the content of the corresponding sections, think about the following questions:

1. For objects that change speed like small balls, _______ should be used to describe the speed of their movement.

Reference answer: average speed

2. Can the average speed accurately reflect the speed of an object's movement? If Xiao Ming rides to school and stops for a period of time, should this period of time be included in calculating his average speed?

Tip: The average speed is used to roughly describe the average speed of a moving object. If you know the average speed of a variable-speed moving object, you can only know roughly how fast it moves, but you cannot know its movement accurately. When calculating the average speed, the total distance is divided by the total time taken to travel this distance, so the time spent in this distance should also be included in the calculation.

Reference answer: The average speed cannot accurately reflect the speed of the object's movement and should be included in the calculation.

3. The speed sign on the expressway is marked with "speed limit 100km/h". Does it refer to the instantaneous speed?

Tip: The speed of a moving object at a certain moment is called instantaneous speed. The speed of vehicles on the highway cannot exceed the speed limit at any time, so the "speed limit 100km/h" marked on the road sign refers to the instantaneous speed.

Reference answer: yes

Knowledge summary

1. Average speed

1. Concept: Indicates the speed of a moving object in _______________.

2. Features: Reflects the speed of movement of objects in _______________.

3.Formula:______.

2. Instantaneous speed

1. Concept: The speed of a moving object at ________.

2. Features: Reflects the speed of movement of an object during ________ or ________.

3. Special case: In uniform linear motion, the instantaneous speed at any moment is _____, and the instantaneous speed at any moment and the average speed during the entire motion are _____.

Test point 1 Calculation and application of average speed

[Typical Example 1] (2012 Neijiang High School Entrance Examination) As shown in the picture, the start and arrival times of a car and the corresponding odometer (the disk indicates kilometers) are displayed. It can be seen from the figure that the mileage traveled by the car is ____km and the average speed is ____m/s. (Keep one digit after the decimal point)

[Concise Lecture and Analysis] This question tests the calculation of speed. As can be seen from the picture, the car is running

The time is: t=12:25-9:25=3h; the distance traveled by the car is s=27582km-27462km=120km; the speed of the car is: ______=40km/h=11.1m/s.

Answer: 120 11.1

[Method Summary] The methods to solve this type of problem are as follows:

(1) Find the distance, time or speed based on the information provided by the picture, and determine the physical quantity to be obtained;

(2) Select an appropriate formula or deformation formula;

(3) Substitute the data to calculate the answer.

1. Which of the following statements about average speed is correct ()

A. Average speed is a physical quantity that reflects changes in the position of an object

B. The average speed can only generally reflect the speed of an object’s movement.

C. The average speed can accurately reflect the speed of a moving object at a certain moment.

D. The average speed can accurately reflect the speed of a moving object during a certain distance.

[Analysis] Choose B. The average speed is a rough reflection of the speed of a moving object at a certain distance or within a certain period of time. It cannot accurately reflect the speed of a moving object at a certain moment or a certain distance.

2. The fable "The Tortoise and the Hare" says: The tortoise and the hare started from the starting point at the same time. The hare fell asleep proudly after far surpassing the tortoise, as shown in the picture. When it woke up, it found that the turtle had quietly crawled across the finish line, and regretted it endlessly. in the whole process ( )

A. The rabbit always runs faster than the tortoise

B. The tortoise always runs faster than the hare

C. The average speed of rabbits is large

D. The average speed of the turtle is large

[Analysis] Choose D. This question tests the understanding of average speed. The rabbit runs faster than the tortoise before going to sleep; when the rabbit is sleeping, it is stationary and its speed is zero, while the tortoise is moving, so the tortoise runs faster than the rabbit at this time; the average speed is equal to the distance traveled by the object and the distance passed by it The ratio of the time taken for a segment of the journey, that is, ______. The tortoise and the hare travel the same distance, but the total time taken by the tortoise is shorter than that by the hare, so the average speed of the tortoise is greater.

Find out where you tend to go wrong?

It is 600 m from home to school. Li Ming walked half the distance at 2 m/s and the next half at 3 m/s. What was his average speed throughout the entire distance?

[Students’ own answers]

Solution: Li Ming’s average speed v=2m/s+3m/s/2=2.5 m/s.

Answer:2.5 m/s

[Teacher Comments]

①The calculation process has necessary text descriptions, basic formulas can be used, and there are units after the numbers, which is good.

②The understanding of average speed is incorrect. The average speed is not the average speed.

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Update Time: 2024-10-20

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