DSE Physics | Projectile Motion: a ten-section tutor guide (resolution, equations, a 45-minute plan)

2026-11-05

Students panic at any curved path because they treat a projectile as one new formula to memorise rather than two independent straight-line motions. This lesson starts from a ball thrown horizontally off a 20-metre platform at 8 m/s, has students compute the 2.02-second flight and 16.2-metre range, then asks why the horizontal direction needs no gravity at all.

Objectives — Students can: (1) resolve projectile motion into constant horizontal velocity and constant vertical acceleration, stating that gravity acts vertically only; (2) work backwards from height to flight time and forwards to range, writing every equation used; (3) judge whether an answer is reasonable in size, direction and unit.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Vector resolution	How is a slanted motion split in two?	Sine and cosine give the two components
Equations of motion	Which equation avoids time?	v squared equals u squared plus 2as
Sign convention	With up positive, what is acceleration?	Minus 9.81 metres per second squared

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Open with a demonstration	Release one ball horizontally and one from rest at 20 m; both land together	Students predict, then describe what they saw
6-14 min	Split into two columns	Left column constant speed, right column constant acceleration; write g once	Students draw the two columns
14-26 min	Height gives the time	Use s equals ut plus half at squared with vertical u zero, so t is 2.02 s	Students compute and write the unit
26-34 min	Time gives the range	Horizontal speed is unchanged, so range is 8 times 2.02, about 16.2 m	Students finish the second part and check the size
34-40 min	An angled launch adds one step	Resolve the initial velocity, stress that both sides share one time	Students do the resolution only
40-45 min	Exit check	Collect three answers, read but do not reteach	Work alone

03 Teaching script (read it out)

Scene 1: Demonstrate before any formula

The two errors that survive teaching are that heavier objects land first and that horizontal speed delays the fall. One release from a 20-metre platform overturns both in a second, which no amount of formula writing achieves.
Do not write equations straight away. Ask first what differs between the two balls vertically. When students answer nothing, they have accepted the premise of resolution.

Students do: predict, watch, then write one line saying the vertical motions are identical.

Scene 2: Two columns: g on one side only

Draw two columns, horizontal on the left and vertical on the right, and write g into the right column only, then circle it. Repeat that gesture three times, because students still substitute 9.81 horizontally in exams.
Fill the knowns: vertically u is zero, s is 20, a is 9.81; horizontally u is 8 and a is zero. With both columns filled, half the question is already done.

Students do: copy the table and mark the only column with acceleration.

Scene 3: Solve for time and let students press the calculator

Write s equals ut plus half at squared, substitute 20 equals half times 9.81 times t squared, so t is the square root of 40 over 9.81, which is 2.02 s. Students must press this themselves because 2h over g and h over 2g get confused.
Then ask what happens to the time if the height quadruples; the answer is double, and that square-root relationship recurs in past papers.

Students do: compute with the unit, then answer the quadruple-height question.

Scene 4: Range: time is the bridge

Horizontal speed never changes, so range is 8 times 2.02, which is 16.2 m. The teaching point is not the multiplication but the sequence: the vertical direction sets the time, and the same time is reused horizontally.
Compare 4 m/s and 8 m/s from the same platform: the time stays 2.02 s while the range doubles from 8.1 m to 16.2 m. Seeing speed double with time fixed proves the two lines are independent.

Students do: fill a three-row table of speed, time and range, and name the column that stayed constant.

Scene 5: An angled launch is one extra step

An angled question only adds resolution: split u into u cos theta and u sin theta. Vertically the same equations apply, horizontally the speed stays constant, and both use one t.
For one launch speed the 45-degree range is largest at about u squared over g, which is 6.5 m at 8 m/s, shorter than the 16.2 m horizontal throw because there the whole speed was horizontal. That detail stops students from reciting that 45 degrees is always furthest regardless of launch height.

Students do: do the resolution only, then explain why an angled throw can travel less far.

04 Question types and worked steps

Type	Example	Steps	Answer
Horizontal launch	Find flight time and range from a platform	Vertical gives t, horizontal multiplies by t	Two equations with units
Angled range	Find range from speed and angle	Resolve, solve vertical time, multiply by the horizontal part	Full substitution, degrees
Impact velocity	Find speed and direction on landing	Horizontal unchanged, vertical is gt, then combine	Give the angle to the horizontal
Graph reading	Decide which line is a projectile	Horizontal displacement is linear, vertical is curved	Justify with gradient

Sanity check (teach this): Sanity checks: (1) a 20-metre fall takes about 2 seconds, so 0.2 or 20 signals a square-root slip; (2) the impact vertical speed is about 19.8 m/s, and being larger than the 8 m/s launch is normal; (3) the range cannot exceed launch speed times total time; (4) the 45-degree maximum applies only to level ground, so from a platform the best angle is smaller; (5) every answer needs a unit, and a velocity needs a direction.

05 Independent practice (3 questions)

Question	Answer	What to watch
Platform throw	From 20 m at 8 m/s find time and range	Check 2.02 s and 16.2 m
Resolution	Split 20 m/s at 30 degrees	Check both components and the angle
Impact speed	Find the landing speed for the same case	Check the combination and direction

06 Common mistakes and what to say

Mistake	Student writes	Say this
Gravity used horizontally	Writing a equals 9.81 in the horizontal column	“Nothing pushes it sideways. The g you circled belongs to the right column only.”
Square root misplaced	Getting the square root of h over 2g instead of 2h over g	“Your time is half of mine. Read the numerator before you press the root key.”
Two different times	Using the total time horizontally and half of it vertically	“One ball, one landing moment. Both columns share the same t.”
No direction given	Reporting a landing speed of 19.8 m/s only	“Velocity is a vector. Say downward, or give the angle.”

07 Homework (paper, section and question numbers)

Redo this chapter — All three practice items with equations and units
Physics index — Other chapters and question types
All prep guides — The same ten sections across subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
Resolution	Constant horizontally, accelerating vertically	Cannot state it, redo act two
Time	From the height by square root	Wrong time, redo act three
Direction	Velocity needs a direction	No direction, redo practice three

09 Tutor FAQ

Must students memorise formulas?

Only three: v equals u plus at, s equals ut plus half at squared, and v squared equals u squared plus 2as.

What about air resistance?

Ignore it in calculations; in discussion questions it explains why real ranges fall short.

Is 45 degrees always best?

Only from level ground; launched from a platform the optimum is smaller.

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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.