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Specific physical meaning
Phase velocity: the velocity of equal phase surface (e.g. crest surface or trough surface) of single frequency sinusoidal electromagnetic wave propagating in the medium, v = C / N, C is the speed of light in free space, and N is the refractive index of the medium to the electromagnetic wave of this frequency
Group velocity: the velocity of the composite signal of many sinusoidal electromagnetic waves with different frequencies propagating in the medium
The amplitude and phase of sine waves with different frequencies are different. In dispersive media, the phase velocity is different, so the shape of the synthesized signal will change in different spatial positions. Group velocity is a propagation velocity representing energy
Why describe the speed of the change in the direction of linear velocity,
Instead of describing the magnitude of linear velocity or angular velocity?
First, centripetal acceleration is acceleration;
Secondly, acceleration describes the speed of speed change, here the speed, including size and direction
In linear motion, the direction of velocity is invariable, so we focus on the speed of velocity change;
In curvilinear motion, if the magnitude and direction of velocity change at the same time, the concept of acceleration is fully embodied here;
In uniform circular motion, the magnitude of velocity (i.e. linear velocity) is invariable, so we only need to discuss the change of velocity direction. Therefore, centripetal acceleration (in non-uniform circular motion, centripetal acceleration is the component of acceleration along the direction pointing to the center of the circle) describes the speed of change of linear velocity direction, because the magnitude of velocity is invariable
In addition, the angular velocity is constant in uniform circular motion
What is the physical meaning and definition of frequency
Frequency is the number of vibrations completed per unit time, and it is the quantity to describe the frequency of reciprocating motion of vibrating objects. It is often expressed by the symbol f or V, and the unit is seconds-1. In memory of the contribution of German physicist Hertz, people named the unit of frequency Hertz, or "Hertz" for short
Why is speed not proportional to distance time in uniform motion
A uniform motion is a motion in which the speed remains constant
If the speed is constant all the time, it can't be proportional to the distance or time
Only that the distance is proportional to time: the longer the speed is, the longer the distance is
Speed doesn't change, so it's not proportional
Relationship between velocity and time of physical linear motion
It is known that o.a.b.c is four points on the same straight line. The distance between AB is L1BC, which is L2. An object starts from O and accelerates through three points ABC AB.BC The time of OA is equal to the distance of OA
Let the acceleration be a, the time for OA is t ', and the time for AB and BC is t
L1=at't+(at^2)/2.(1)
L2=a(t'+t)t+(at^2)/2.(2)
By subtracting the two formulas, t = under the radical [(l2-l1) / a]... (3)
By bringing (3) back to (1), we can get: t '= (3l1-l2) / [a (l2-l1)] (4)
OA=(a*t'^2)/2.(5)
If (4) is brought into (5), OA = (3l1-l2) ^ 2 / [8 (l2-l1)]
Note: T ^ 2 is the second power of T
How can the increment of velocity be equal in the same time?
Because the acceleration doesn't change
Unless you think that the closer an object is to the earth, the more gravitational it is, it's hard to understand
1. The total length of the railway from Xi'an to Baoji is 170km. T797 departs from Xi'an at 6:00 p.m. and arrives at Baoji Station on time at 19.40 p.m. when does y797 pass through Guozhen? (known as 20km from Guozhen station to Baoji Station)
It took Xiaoming 30 seconds to ride 100 meters before school and 20 seconds to ride 80 meters after school. What's the average speed of Xiaoming on the whole road?
3. It takes 4 seconds for an object to move along a straight line, and the average speed is 30 m / s. what is the average speed of the object in the whole process when the average speed of the second half of the distance is 20 m / S?
Problem solving format:
Known:
Ask for:
A:
You're not taking a test, are you
Known: S = 170 t = 19.40-18 = 1 hour 40 minutes
How long does it take to pass through Guozhen
v=s/t=170/100=1.7km/min s1/v=20/1.7=11.8 19.40-11.8fen=19.28
A: it's about 19:28 after Guozhen
Known: s total = 100 + 80 = 180m, t total = 30 + 20 = 50s
What is the average speed of Xiao Ming on the whole road?
V=S/T=180/50=3.6M/S
A: the average speed of Xiaoming on the whole road is 3.6m/s?
Known:
Ask for:
Total distance S = 30 * 4 * 2 = 240m T2 = 120 / 20 = 6S total time t = 4 + 6 = 10s so v = 240 / 10 = 24m / S
answer
Eighth grade physics about speed and distance
What's Xiaoming's average speed in the first 50m, the second 50m and the whole race
What's the formula
It is known that s = 100m, t front = t back = 50m, t front = 6S, t back = 7S, t = 6S + 7S = 13s
Find: V before, V after, V
V front = s front / T front = 50M / 6S = 8.3m/s
V after = s after / T after = 50M / 7S = 7.1m/s
v=S/t=100m/13s=7.7m/s
A: the average speed of the first 50 meters is 8.3m/s, the average speed of the last 50 meters is 7.1m/s, and the average speed of the whole journey is 7.7m/s
In physics, we use "distance per unit time" to express speed. After discussing with her classmates, Wang Fang knows that speed can also be expressed by "time per unit distance", and the unit is s | M
A. 1s | m is faster than 1m | s
B. 0.2 s | m is faster than 5 m | s
C. 100 m | s is faster than 10 s | M
C. 0.1s | m is faster than 10m | s
The four options are relatively fast and slow, so the unit is converted to m / s, how many meters per second, the big one is fast, it is clear at a glance. A option, 1s / M reciprocal or 1, the same B option, 0.2 reciprocal or 5, the same C option, 10 reciprocal, 0.1m/s is smaller than 100m / s, C option is correct, D option, 0.1 reciprocal or 10, the same to sum up, C is correct
The following statements about instantaneous velocity and average velocity are correct ()
A. If the instantaneous velocity of an object is equal to zero at any time in a certain period of time, then its average velocity in this period of time must be equal to zero B. if the average velocity of an object in a certain period of time is equal to zero, then its instantaneous velocity at any time in this period of time must be equal to zero C The average speed in any period of time in uniform linear motion is not equal to its instantaneous speed at any time D. the average speed in any period of time in variable speed linear motion is not equal to its instantaneous speed at any time
A. If the instantaneous velocity of an object is equal to zero in a certain period of time, it means that it is stationary, then its average velocity in this period of time must be equal to zero, so a is correct; B. if the average velocity of an object in a certain period of time is equal to zero, it means that its displacement is zero, but it can not mean that it is stationary, such as the movement of an object
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