Physics 8 Lesson 3: Uniform motion Irregular motion
1. Theoretical summary
1.1. Even motion
Uniform motion is motion in which velocity does not change with time.
+ Example:
1.2. Uneven movement
Irregular motion is motion in which the velocity varies with time.

Eg: The motion of a car on the road, sometimes fast and slow, varies with time, ⇨ that motion is nonuniform.
1.3. Average speed of irregular motion
– The average speed of an irregular motion over a distance is calculated by the formula: \({v_{tb}} = \frac{s}{t} \)
In which:
Attention: When it comes to average speed, it must be clearly stated on which road segment the average speed is calculated because on different road segments, the average speed can be different.
1.4. Solution method
a) Calculate the average speed of the irregular motion
– Recipe: \({v_{tb}} = \frac{{{s_1} + {s_2} + … {s_n}}}{{{t_1} + {t_2} + … {t_n}}} \)

In which: s_{first,} S_{2}…S_{n} and t_{first}, t_{2}…tn are the distances covered and the time to go all the way.
Attention: Average velocity is completely different from average velocity: \({v_{tb}} \ne \frac{{{v_1} + {v_2} + … + {v_n}}}{n} \)
b) Method of solving the problem by graph
– Usually choose the origin to coincide with the starting point of either motion. Choose the vertical axis as Ox (representing the distance traveled), the horizontal axis as Ot (representing time).
– The graph is a straight line that can pass through the origin O or not, depending on the way we choose the datum and the time.
– Write an equation of the path of each motion of the form: x = x_{o} + s = x_{o} + v(t – t_{o})
In which:
– Graph each movement. Based on the intersection of the graphs to find the time and place where the movements meet.
2. Illustrated exercise
2.1. Form 1: Determine the average speed of the object
A car goes up a hill with a speed of 16 km/h, when it comes back down the same slope, the car moves twice as fast as it does up the hill. What is the average speed of the car on both the uphill and downhill segments?
Direction explain
Let s be the length of the ramp
\({t_1} = \frac{s}{{16}} \) it’s uphill time
\({t_2} = \frac{s}{{32}} \) it’s down time
The average speed of the car in both segments is:
\(v = \frac{{2s}}{{{t_1} + {t_2}}} = \frac{{2{\rm{s}}}}{{\frac{s}{{16}} + \frac{s}{{32}}}} = 21.33\,km/h\ \)
2.2. Form 2: Determine the speed of the object
An airplane carrying passengers flies between two cities A and B. When it’s downwind, the flight time is 1h30′, and when it’s upwind, the flight time is 1h45′. Assume that the wind speed remains constant at 10 m/s. What is the speed of the plane in the absence of wind?
Direction explain
Let v be the speed of the plane, v_{g} is the speed of the wind.
t_{first}, t_{2 }are the downwind and upwind time, respectively.
t_{first} = 1h 30′ = 5400 s
t_{2} = 1h 45′ = 6300 s
Because the distance traveled by the plane in the downwind and against the wind is the same
t_{first}(v + v_{g}) = t_{2}(v – v_{g})
\(\frac{{{t_1}}}{{{t_2}}} = \frac{{v + {v_g}}}{{v – {v_g}}} \)
\(\Rightarrow \frac{{5400}}{{6300}} = \frac{{v + 10}}{{v – 10}} \)
⇒ 5400.(v – 10) = 6300.(v + 10)
⇒ 900.v = 63000 + 54000 = 117000 ⇒ v = 130 m/s = 468 km/h
3. Practice
3.1. Essay exercises
Question 1: An airplane carrying passengers flies between two cities A and B. When it’s downwind, the flight time is 1h30′, and when it’s upwind, the flight time is 1h45′. Assume that the wind speed remains constant at 10 m/s. What is the speed of the plane in the absence of wind?
Verse 2: A cyclist travels at a speed of 20 km/h for the first half of the distance. Calculate the person’s speed for the remaining half of the distance. Assume that the average speed for the whole journey is 23 km/h.
Question 3: Motorcyclists on road AB. The first half of the way he travels at a speed of 30 km/h. In the other half of the time travel at 25 km/h. In the end he went at a speed of 15 km/h. Find the average speed for the whole road segment AB.
Verse 4: A motorboat is moving uniformly on the river. The speed of the boat when downstream is 20 km/h and when upstream is 15 km/h.
a) If the boat does not start, what is the distance traveled by the boat with the current in 30 minutes?
b) Assuming the water is still and the boat starts up, what is the speed of the boat at that time?
3.2. Multiple choice exercises
Question 1: When it comes to the speed of means of transport such as motorbikes, cars, trains, airplanes, etc., people talk about it.
A. instantaneous velocity.
B. average velocity.
C. the maximum achievable speed of that vehicle.
D. minimum achievable speed of that vehicle.
Verse 2: Uniform motion is motion with magnitude of velocity
A. remains constant throughout the time the object is in motion.
B. remains constant throughout the journey.
C. is always kept constant, while the direction of velocity can be changed.
D. Sentences A, B, and C are all correct.
Question 3: Which of the following is uniform motion?
A. Skier downhill.
B. The 100meter runner is reaching the finish line.
C. The plane flies from Hanoi to Ho Chi Minh.
D. None of the above motions is uniform.
Question 4: A person travels a distance s_{first} with speed v_{first} over_{first} second, travels the next distance s2 with speed v_{2} over_{2} second. What formula is used to calculate this man’s average speed over both distances s .?_{first} and s_{2}?
A. \({v_{tb}} = \frac{{{v_1} + {v_2}}}{2} \)
B. \({v_{tb}} = \frac{{{s_1} + {s_2}}}{{{t_1} + {t_2}}} \)
C. \({v_{tb}} = \frac{{{s_1}}}{{t_1}}} + \frac{{{s_2}}}{{{t_2}}} \)
D. Both B and C are correct
4. Conclusion
Through this lesson, students will be familiar with knowledge related to Uniform motionIrregular motion along with related exercises in many levels from easy to difficult…, you need to understand:
 Give examples of common irregular motions.
 Use to calculate average speed on a road segment.
 The statement defines uniform motion and gives examples of uniform motion.
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