Average Speed and Journey Time

What you’ll learn

Relate total distance to total elapsed time.

ភ្ជាប់ចម្ងាយសរុបទៅនឹងរយៈពេលសរុប។

Meaning and use

Average speed is total distance divided by total elapsed time. It is not generally the arithmetic mean of several speeds. Include stops if the question asks for the whole journey’s elapsed time. Speed is a scalar; velocity also describes direction.

ល្បឿនមធ្យមស្មើចម្ងាយសរុបចែករយៈពេលសរុប។ មិនមែនតែងស្មើមធ្យមភាគល្បឿននីមួយៗទេ។ បើសួរដំណើរទាំងមូល ត្រូវរួមពេលឈប់។

Formulas

Average speed

\[v_{\mathrm{avg}}=\frac{d_{\mathrm{total}}}{t_{\mathrm{total}}}\]

Constant-speed rearrangements

\[d=vt,\qquad t=\frac dv\]

Symbols and units

Symbol	Meaning	Unit or restriction
vavg, v	Average speed, or a constant speed	m/s or km/h
d	Distance travelled	m or km matching the time unit
t	Elapsed time	s or h

Conditions and limits

• Elapsed time must be positive; t = d/v requires positive speed.
• d = vt uses a constant speed or an average speed for the stated interval.
• កុំច្រឡំចម្ងាយជាមួយបម្លាស់ទី។

Worked examples

A walking speed

A learner walks 120 m in 60 s.

1. Use \(v=120/60\).
2. Calculate \(v=2\) m/s.

2 m/s

ចែកចម្ងាយសរុបដោយពេលវេលាសរុប។

Including a stop

Travel 60 km in 1 h, stop for 0.5 h, then travel 40 km in 1 h.

1. Total distance 100 km.
2. Total elapsed time 2.5 h.
3. Average speed \(100/2.5=40\) km/h.

40 km/h

ពេលឈប់ត្រូវរាប់ក្នុងរយៈពេលដំណើរទាំងមូល។

Common mistakes

Instead of	Use	Why
Average speed is always (v₁ + v₂)/2.	Use total distance divided by total time.	A simple mean works only under appropriate equal-time conditions.
120 m / 2 min = 60 m/s.	120 m / 120 s = 1 m/s.	Convert minutes before reporting m/s.

Practice

Solve these before revealing the worked solutions. Include units where relevant.

សាកល្បងដោះស្រាយមុនបើកមើលចម្លើយ។ សូមបង្ហាញជំហាន និងឯកតាដែលពាក់ព័ន្ធ។

1. Distance 300 m in 100 s: find average speed.
2. At constant 5 m/s for 20 s, find distance.
3. How long does 150 m take at constant 3 m/s?

Answers and steps

Show solutions / បង្ហាញដំណោះស្រាយ

1. 3 m/s

   300/100 = 3.

2. 100 m

   5 × 20 = 100.

3. 50 s

   150/3 = 50.

Quick recap

• Use total distance and total elapsed time.
• Convert time units carefully.
• Distinguish speed from direction-dependent velocity.