Shanghai Science Edition 8th Grade Physics

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"Fast and Slow" The World of Movement PPT Courseware

1. How do you compare the speed of an object’s motion?

A track and field race is going on, how do we judge who is running faster?

Comparison method:

(1) Same time and distance. (audience)

(2) The same distance and time. (referee)

In physics, we use "the same time versus distance" and introduce a physical quantity to represent the speed of an object's movement, which is velocity.

2. Speed

(1) Definition: In physics, the distance traveled per unit time is called speed.

That is: speed = distance / time

If we use the symbol v to represent speed, s to represent distance, and t to represent time, the above formula can be written as:

The physical meaning of speed: In physics, speed is used to express how fast an object moves.

(2) Unit of speed

The unit of speed is a composite of the units of distance and time. If the unit of distance is meters, the unit of time is seconds.

The main unit of speed is "meter/second", which is expressed in symbols as m/s or m·s and pronounced as meters per second.

Commonly used units include "kilometer/hour", which is represented by the symbol km/h.

3. Linear motion

(1) Uniform linear motion

The linear motion in which an object moves at a constant speed is called uniform linear motion.

(2) Variable speed linear motion

Linear motion with changing speed is called variable speed linear motion.

In nature, most motion is linear motion with variable speed.

In variable speed linear motion, distance and time are corresponding, and the speed we calculate is the average speed of the segment.

4. Consolidate practice

1. A train moves in a straight line at high speed. It covers a distance of 150m in 5s. Its speed in the first 2s is _____m/s. Combined _____km/h, the distance traveled is _____m.

2. A person rides a bicycle along a straight road. The distance traveled in the first s is 2m, the distance traveled in the second s is 3m, and the distance traveled in the third s is 4m. Find:

①The average speed in the first 2s is _____m/s. ②The speed in the 3rd second is ___m/s.

five. summary

Through the study of this section, we know how to compare the speed of an object's movement, understand the requirements for solving physical calculation problems, know the concepts and calculation formula units of speed, and can perform simple calculations.

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Update Time: 2024-09-30

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