Dạy lớp 10.2 thứ 7 ngày 26-9

Main ideas / lessons conveyed

1) Classroom workflow (homework check + platform access)

• The teacher checks students’ homework from the previous day, which included a multiple-choice test on Topic 3: displacement and distance traveled.
• Students are instructed to:
  • Type in the chat to ask which homework questions need answers.
  • Confirm they have logged into Azota and completed assignments there.
• If links aren’t visible, the teacher provides an Azota QR code.

2) Topic 4: Speed vs. Velocity (core physics concepts)

Key conceptual distinction

• Speed (scalar): how fast something moves (no direction).
• Velocity (vector): how fast something moves and in what direction.

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Speed (Roman part I)

Average speed definition

• Average speed is found by dividing total distance traveled by total time.
• For motion split into stages:
  • Students may incorrectly average the stage speeds (explicitly called out as wrong).
  • Correct method: [ \text{average speed}=\frac{S_1+S_2+\cdots+S_n}{T_1+T_2+\cdots+T_n} ]

Comparing which movement is faster (two approaches)

• If distance is the same, compare time: less time → faster.
• If time is the same, compare distance: greater distance → faster.
• This connects to the “distance per unit time” idea (using 1 second as a reference).

Units and conversions

• Everyday: km/h
• SI standard: m/s

• Conversion rules: [ \text{km/h} \to \text{m/s}:\ \text{divide by }3.6 ] [ \text{m/s} \to \text{km/h}:\ \text{multiply by }3.6 ]

• Examples:

  • 36 km/h → 10 m/s
  • 54 km/h → 15 m/s

Instantaneous speed

• Definition: speed at an exact moment.
• Practical measurement: a speedometer shows instantaneous speed.
• Theoretical link:
  • Instantaneous speed ≈ average speed over an extremely small time interval.

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Velocity (Roman part II)

Why direction matters

• Distance alone can’t determine final position without direction.
• Velocity provides enough information: displacement over time + direction.

Average velocity definition

• Average velocity is a vector: [ \vec{v}_{avg}=\frac{\text{displacement}}{\text{time}}=\frac{\vec{d}}{T} ]

• In delta form: [ \vec{v}_{avg}=\frac{\Delta \vec{d}}{\Delta t} ]

• Direction property:

  • The velocity vector points in the same direction as displacement (based on sign conventions).

Instantaneous velocity

• Like instantaneous speed, but includes direction:
  • Instantaneous → at a specific point in time
  • Average → over a time interval
• Conceptual idea:
  • Instantaneous velocity ≈ average velocity over a very small (\Delta t).

Special relation: when speed = magnitude of velocity

• In straight-line motion without changing direction: [ \text{speed} = |\text{velocity}| ]

• If direction changes, speed and velocity differ.

Straight-line motion with sign (projection method)

• Use: [ v=\frac{x_2-x_1}{t} ]

• Sign interpretation:

  • Positive (v): motion in (+\text{Ox}) (if (x_2 > x_1))
  • Negative (v): motion in (-\text{Ox}) (if (x_2 < x_1))
• Teacher’s instruction: for rectilinear motion, scalar form with sign can be used instead of full vector notation.

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3) Exercise practice: distinguishing speed vs velocity

The teacher repeatedly uses word problems and emphasizes typical mistakes:

• Average speed uses: total distance / total time
• Average velocity uses: displacement (net change) / total time
• Displacement can be smaller than total distance (especially with direction reversals), so:
  • speed can be greater than the magnitude of velocity

Examples used:

• Tri-part route (home → school → supermarket)
• Swimming in a pool:
  • Out-and-back returns to starting point:
    • displacement = 0 → velocity/average velocity = 0
    • but distance > 0 → average speed > 0
• Ant around a circular cup:
  • One full circle:
    • distance traveled = (2\pi R)
    • displacement = 0

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4) Further applied problems (sound in water, car motion, multi-stage motion)

Sonar depth problem (Lesson 162)

• Sonar measures time for sound to go down and reflect back.
• Sound travel distance = 2H (down to seabed and back up).
• Given:
  • time = 0.13 s
  • speed of sound in water = 1500 m/s
• A correction was made:
  • Some groups incorrectly used (1500\times 0.13) without halving.
• Correct approach: [ 2H = 1500 \times 0.13 \Rightarrow H = 97.5\ \text{m} ]

Car position measured at equal time intervals (Lesson 163)

• Average speed over interval(s):
  • Use distance traveled between corresponding position readings.
• Emphasis:
  • Correctly map which “seconds” correspond to which position changes (e.g., “third second” means from (t=2) to (t=3)).

Multi-stage average speed / velocity (Lessons 164–167)

• Common formula checks:
  • Average speed: total distance / total time
  • Average velocity: total displacement / total time
• Convert units properly (km/h ↔ m/s ↔ hours ↔ seconds) when combining legs.

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5) Velocity addition (vector addition of velocities)

Concept: relative velocity framework

• Standard notation uses three objects:
  • Object 1 moving relative to object 2
  • Object 2 moving relative to object 3
  • Then object 1 relative to object 3

• Core rule: [ \vec{V}{13}=\vec{V} ]}+\vec{V}_{23

• Conventions:

  • 1 = object of interest
  • 3 = reference/stationary object (often “ground/shore/road”)
  • 2 = intermediate moving frame (water, wind, train-car frame, etc.)

Special cases taught

• Same direction (collinear): [ V_{13} = V_{12} + V_{23} ]

• Opposite directions (collinear): [ V_{13} = |V_{12} - V_{23}| ]

• Perpendicular directions: [ |V_{13}|=\sqrt{V_{12}^2+V_{23}^2} ]

Worked examples

• Canoe and river current:
  • downstream vs upstream: add vs subtract
  • crossing perpendicular to bank: combine perpendicular components → resultant via Pythagoras
• Train + ticket inspector:
  • interpret relative motion with direction and the relative-velocity equation
• Rescue team canoe downstream/upstream:
  • forward/back motion uses add/subtract relative to water/current speed
  • time computed from net speed in each direction

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Methodology / instruction-style bullet points (as presented)

A) Correct method for average speed (multi-stage)

• Do not average stage speeds directly.
• Instead:
  • Sum all distances: (S_1+S_2+\cdots+S_n)
  • Sum all times: (T_1+T_2+\cdots+T_n)
  • Compute: [ v_{avg}=\frac{S_1+S_2+\cdots+S_n}{T_1+T_2+\cdots+T_n} ]

B) Converting speed units

• km/h → m/s: divide by 3.6
• m/s → km/h: multiply by 3.6
• Examples:
  • 36 km/h → 10 m/s
  • 54 km/h → 15 m/s

C) Instantaneous speed/velocity definition (conceptual)

• Instantaneous quantity ≈ average quantity over a very small time interval: [ v_{inst}\approx \frac{\Delta s}{\Delta t}\ \text{as}\ \Delta t\to 0 ]

• Instantaneous speed from a speedometer.

• Instantaneous velocity includes direction.

D) Straight-line velocity with sign (projection)

• Choose axis (Ox) aligned with motion.

• Use: [ v=\frac{x_2-x_1}{t} ]

• Interpret sign:

  • (v>0): +Ox direction
  • (v<0): −Ox direction

E) Relative velocity (vector addition framework)

• Use: [ \vec{V}{13}=\vec{V} ]}+\vec{V}_{23

• Then choose:

  • Same direction → add magnitudes
  • Opposite direction → subtract magnitudes
  • Perpendicular → use Pythagoras on magnitudes/components

F) Sonar depth correction

• For sound reflecting off seabed:
  • travel distance is twice depth (down + back)
• Use: [ 2H = v_{sound}\,t \Rightarrow H=\frac{v_{sound}\,t}{2} ]

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Speakers / sources featured (explicitly identifiable)

• Teacher / instructor (main speaker; unnamed)
• Students (mentioned or spoken to):
  • Ho Hanh Anh
  • Nguyen Duc Hieu
  • Hương B. (H-b)
  • Qu Lưu Quang Minh
  • Bao Ngọc
  • Tiến (called during an exercise discussion)
  • Nguyễn Phong
  • Khanh Ha
  • Trung Hải
  • Minh Anh
  • Thanh Phong
  • Thuy Phuong
  • Vu Khanh (called as “Vu Khanh, turn on your mic”)
  • Nguyen Minh Anh (appears in the flood/rescue section)
  • Other mentions: The Khang; Hanh Anh
• Azota (platform)
• Speedometer (device)