An object moving at variable speed displacement The ratio to time is not constant. At this time, we can use a speed to roughly describe the speed of the object in this period of time. This speed is called the average speed. (△ x=displacement △ t=passing through this section displacement Time spent) Other calculation formulas
2 × V1 × V2 ÷ (V1+V2)=average speed. (Average speed of the first half of the journey V1, average speed of the second half of the journey V2)
In the constant speed linear motion, the average speed can also be calculated by (V0+Vt) ÷ 2. At this time, the average speed also represents the middle time of the time used to pass this displacement Instantaneous velocity 。 But if it is Uniform speed movement , then there is another formula=(initial speed+final speed)/2 (1) It reflects the average speed of object movement in a period of time, which corresponds to a period of displacement or a period of time.
(2) On Variable speed linear motion In, the average speed is related to the selected time or displacement. The average speed in different time periods or on different displacements is generally different. It is necessary to indicate which time period or displacement period the average speed is calculated. It is meaningless not to indicate the average speed of the corresponding process. (3) The average speed is a vector, and its direction is the same as the displacement direction occurred in a period of time Δ t, but not necessarily the same as the direction of motion.
(4) On uniforn variable rectinear motion The instantaneous speed at the middle position is greater than the instantaneous speed at the middle time. deduction:
The speed at intermediate time is V t , intermediate displacement Vs, initial speed V zero , the final speed is V p , the displacement of the whole section is S
Vp=Vo+at
Vt=Vp+1/2at
Vt=(Vo+Vt)/2=average speed
V p two —V zero two =2aS
V s two —V o two =2a*（1/2）*S=（1/2）*（V p two —V zero two ）
V s =√[（V p two +V zero two ）/2]
Also ☒ The speed at the middle position (Vs) is the square average, and the speed at the middle moment (Vt) is the average mean value inequality It can be seen that the instantaneous speed at the middle position is greater than the instantaneous speed at the middle time. Difference between average speed and average speed
Average rate Not average speed. The average speed is the ratio of the distance an object passes through to the time it takes to pass this distance. It is scalar 。 (The average speed is equal to the average speed in numerical value when it moves in a single direction in a straight line.)
Average rate It is the ratio of distance to time. The ratio cannot be measured. Generally, it is not equal to the average speed. For example, when an object moves around a circle, the time it takes is t, the average speed is 2 π r/t, and the average speed is 0.
Specifically, the average speed refers to the displacement speed of the object in the selected time. In the above example, the object has returned to its original position after t seconds, so its displacement is zero, and the average speed is zero.
