Junior Physics: Forces and Motion Tutor Prep Guide | Average Speed, Distance-Time Graphs, Inertia

2026-11-12

Tutor lesson-prep guides · Junior Secondary Physics

Students divide the numbers on sight, one of them in minutes, and call every sloping line acceleration. This lesson fixes three moves in order: unify the units, read the slope of the section you need, then explain everyday events with inertia. With that order, both long questions and multiple choice stop being guesses.

Objectives — Students can: (1) calculate average speed from distance and time, convert it to kilometres per hour, and explain why units must match; (2) read constant speed, rest and changing speed from a distance-time graph and use the slope of one section only; (3) explain a passenger lurching forward when a bus brakes using Newton first law, and separate balanced forces from action-reaction pairs.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Units of time and length	How many minutes is one second?	One sixtieth of a minute, so convert before dividing
Reading a straight line	What does a horizontal line on a distance-time graph mean?	Position does not change, so the object is at rest
Describing a force	What two things must a force description include?	Size and direction

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Two opening questions	Ask the same distance twice with different units	Write an answer now, compare at the end
6-20 min	Average speed in three steps	Write data, unify units, divide then convert	Compute metres per second and kilometres per hour
20-34 min	Distance-time graphs	Slope for constant speed, flat for rest, downward slope for return	Find the slope of one section
34-46 min	Newton first law	No force, or balanced forces, keeps the state of motion	Classify three everyday cases
46-56 min	Balanced forces versus pairs	Four checks on one body against two bodies	Name the body being acted on
56-60 min	Exit check	Collect two questions, mark at once	Complete independently

03 Teaching script (read it out)

Scene 1: Average speed: unify units first, then divide

The usual mistake is dividing two numbers when one of them is in minutes. Fix the order in the first sentence: write the data, unify the units, and only then divide. Example: a runner covers 100 m in 12.5 s, so average speed is 8.0 m/s; multiplying by 3.6 gives 28.8 km/h, and this conversion is where most long questions lose a mark. Then separate average from instantaneous: average speed uses only the start and end, while a speedometer shows instantaneous speed. Close with direction: velocity has a direction, speed does not, so a question that asks for speed does not need one.

Students do: convert three race times into metres per second and rank them from fastest to slowest.

Scene 2: Distance-time graphs: the slope is the speed, a flat line is rest

Only three motions exist on this graph: a straight slope is constant speed, a horizontal line is rest, a curve is changing speed. A steeper slope is faster and a downward slope means returning to the start. Example: the first 12.5 s cover 100 m, giving a slope of 8.0 m/s, then 7.5 s pass with no change in position, and the line finally slopes back down to zero. Show how to pick points: subtract two readings from the same section, never across sections, which is the second most common error. Give a self-check: sloping up means moving forward, flat means stopped, sloping down means returning. Any answer that breaks these three is a misread line.

Students do: mark the three time sections on a graph and write one sentence on what the object is doing in each.

Scene 3: Inertia and balanced forces: no net force keeps the motion

Newton first law has only two clauses: with no force, or with balanced forces, an object stays at rest or moves at constant velocity in a straight line. Inertia is not a force, so never write that an inertia force pushes the passenger forward. The correct chain is that the passenger keeps the original speed while the bus slows down, so the passenger moves forward relative to the bus. Put the four checks for balanced forces on the board: same body, same line, equal size, opposite direction. A book on a table has weight and normal force as a balanced pair; the book pressing the table and the table pushing the book are an action-reaction pair on two different bodies. Test them with two 50 N arrows: same direction is not balanced, and opposite directions on two bodies are not balanced either.

Students do: sort four everyday situations into balanced forces or action-reaction, and state which body each force acts on.

04 Question types and worked steps

Type	Example	Steps	Answer
Average speed calculation	Find average speed and convert the unit	Write data, unify units, divide, multiply by 3.6	Always give the unit and the direction when velocity is asked
Graph reading	Read speed, rest periods and total distance	Slope first, then flat sections, then add the distances	Use two points from the same section only
Inertia explanation	Explain a passenger lurching forward	Passenger keeps speed, bus slows, relative motion is forward	Never write inertia force
Balanced force check	Decide whether two forces balance	Same body, same line, equal size, opposite direction	Action-reaction pairs act on two bodies and are not balanced

Sanity check (teach this): Sanity check: the whole lesson keeps one order (units, calculation, then direction), so students cannot skip a step; every example carries numbers they can verify at once (100 m in 12.5 s is 8.0 m/s and 28.8 km/h); graph questions must show the two readings used, so the tutor can tell a misread line from an arithmetic slip; the lesson ends with the three-check rule for reading graphs.

05 Independent practice (3 questions)

Question	Answer	What to watch
Unit conversion	Convert three race times into metres per second	Check that units and direction are written
Slope of a section	Find two speeds on a graph that has a rest period	Check that points come from one section
Inertia wording	Explain the bus braking case in three sentences	Check that inertia force never appears

06 Common mistakes and what to say

Mistake	Student writes	Say this
Dividing before unifying units	Mixing minutes with metres	Work in matched units first: seconds with seconds, metres with metres. A wrong unit makes every later step worthless.
Treating inertia as a force	Writing that an inertia force pushes the passenger forward	Inertia is a property, not a force. Write that the passenger keeps the original speed while the bus slows down.
Confusing balanced forces with action-reaction	Calling the weight of a book and its push on the table a balanced pair	Ask which body is acted on: balanced forces share one body, action and reaction sit on two bodies.
Subtracting across graph sections	Using total distance over total time as one section speed	A slope belongs to one section only, so both points must sit on that section.

07 Homework (paper, section and question numbers)

Junior physics chapter index — All chapters for the subject
Prep guide master index — Similar chapters in other subjects
Redo this chapter — Practice questions 1, 3 and 5: unit conversion, graph slope, inertia explanation

08 Exit check (last 5 minutes)

Question	Answer	Decision
Speed and velocity	Velocity has direction, speed does not	Mixing them means replay scene one
Three graph shapes	Up for forward, flat for rest, down for return	A misread line means replay scene two
Four checks for balance	Same body, same line, equal size, opposite	A missing check means replay scene three

09 Tutor FAQ

Should students learn three rearrangements of the formula?

Teach one formula and rearrange it for speed, distance and time; three memorised versions invite confusion.

Is acceleration an S3 topic?

S3 only needs the idea that speed can change and a curve on the graph shows it; the formal equation belongs to S4.

How is inertia linked to mass?

More mass means more inertia, which is why a loaded lorry needs a longer braking distance. That sentence answers most explanation questions.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.