S3 Science — Forces and Motion: Newton First Law and Net Force Lesson Prep Guide

2026-11-15

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Students think speed means force and steady speed means no force. This lesson separates force from motion with a free-body diagram, then uses one acceleration equation for a trolley, a braking car and a graph.

Objectives — Students can: (1) draw a free-body diagram and state Newton first law, that zero net force means unchanged velocity; (2) compute acceleration as change in velocity over time and use net force equals mass times acceleration; (3) read acceleration from the slope of a velocity-time graph and displacement from the area under it, and explain why the two differ.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Speed and units	A runner covers 100 m in 20 s. Average speed?	5 m/s, with the unit written
Adding forces	30 N and 20 N push the same trolley the same way. Net force?	50 N, added because same direction
Weight and mass	Weight of a 60 kg student? Take g as 10	600 N, mass times 10

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	A wall that will not move	Ask students to push the wall for 3 s, then ask whether it moved	State that force is not the same as motion
5-15 min	Free-body diagram	Draw a trolley: 200 N forward, 50 N friction; students write the net force	Write 150 N to the right
15-29 min	The acceleration equations	Work through 0 to 20 m/s in 8 s with 1,200 kg, asking for the next step each time	Name the equation used at each step
29-37 min	Reading a graph	Give a velocity-time graph and ask which part is speeding up	Say slope is acceleration, area is displacement
37-42 min	Independent practice	Three questions on trolley, braking and area; tour asking only for the net force	Draw the net force before writing
42-45 min	Exit check	Collect answers and ask for the net force at steady speed	Answer zero net force

03 Teaching script (read it out)

Scene 1: Inertia: no net force, no change in velocity

Students treat still as no force and moving as force. Start with the wall: it did not move, yet they pushed hard. Then describe a car sliding on a frictionless surface: with no net force it keeps the same velocity forever. It stops only because friction pushes backwards. So the first law is not force makes things move, it is net force changes velocity.

Students state that the wall did not move although they pushed

Scene 2: Acceleration: one equation, three question forms

Acceleration is change in velocity over time, in metres per second squared. From 0 to 20 m/s in 8 s it is 20 minus 0 divided by 8, which is 2.5. Then net force equals mass times acceleration: 1,200 kg times 2.5 gives 3,000 N. If the question gives 200 N of push and 50 N of friction, first write net force as 200 minus 50 equals 150 N, then divide by mass. The order never changes: net force first, acceleration second, numbers last.

Students repeat that net force comes before acceleration

Scene 3: Graphs: slope is acceleration, area is displacement

On a velocity-time graph the horizontal axis is time and the vertical is velocity. A rising line is speeding up, a flat line is steady speed, a falling line is slowing down. Slope is change in velocity over time, so slope is acceleration. Area is velocity times time, so area is displacement. The common error is calling the area an acceleration. Ask students to check units: metres per second times seconds gives metres, so the area must be a length.

Students use units to explain why area is displacement

04 Question types and worked steps

Type	Example	Steps	Answer
Net force then acceleration	Push 200 N, friction 50 N, mass 50 kg. Acceleration?	1) Net force is 200 minus 50, so 150 N. 2) Acceleration is net force over mass. 3) 150 divided by 50	3 m/s2, same direction as the push
Force from a speed change	A 1,200 kg car goes from 0 to 20 m/s in 8 s. Net force?	1) Acceleration is 20 minus 0 over 8. 2) That is 2.5 m/s2. 3) Net force is 1,200 times 2.5	3,000 N
Acceleration from a graph	On a velocity-time graph, 0 to 8 s rises from 0 to 20 m/s	1) Take two points. 2) Slope is 20 divided by 8. 3) Add the unit	2.5 m/s2
Displacement from area	The same graph, 0 to 8 s, is a triangle	1) Area is half times base times height. 2) Half times 8 times 20	80 m, with the unit stated

Sanity check (teach this): Three checks. Direction: always write the direction of the net force, and subtract when two forces oppose. Units: acceleration in m/s2, force in N, displacement in m; a mixed unit is always wrong. Order: net force before acceleration, because dividing 200 N by mass straight away gives an answer that is too large. Numerically, 8 s from 0 to 20 m/s means an average speed of 10 m/s, so the displacement must be 80 m, which cross-checks the triangle area.

05 Independent practice (3 questions)

Question	Answer	What to watch
A 60 kg student stands still in a lift. What is his weight?	600 N	Whether mass and weight were separated
150 N pushes a box 4 m in 3 s. What is the power?	200 W	Whether work of 600 J was found before dividing by time
A velocity-time graph stays at 20 m/s from 5 s to 10 s. What is the acceleration?	0 m/s2	Whether steady speed was misread as acceleration

06 Common mistakes and what to say

Mistake	Student writes	Say this
Force treated as speed	Because there is a force the car must be speeding up	Force changes speed, it does not keep speed
Opposing forces added	Net force is 200 plus 50, so 250 N	Opposite directions means subtract, draw the arrows first
Area called slope	The area under the graph is the acceleration	Check units: metres per second times seconds is metres
Direction left out	Net force is 150 N	How many newtons, and which way? Write both

07 Homework (paper, section and question numbers)

Pressure, upthrust and floating, question 1 — Both need the larger force identified before any formula
Heat transfer and insulation, question 4 — Both ask for a reason stated as an energy flow
Sound and waves, question 2 — Both want units written before substitution

08 Exit check (last 5 minutes)

Question	Answer	Decision
What is the net force on a body moving at steady speed?	Zero	Answering that there is a forward force means force and speed are still confused
Push 200 N, friction 50 N, mass 50 kg. Acceleration?	3 m/s2	Answering 4 m/s2 means the 200 N was used instead of the net force
What does a flat line on a velocity-time graph mean?	Steady speed, zero acceleration	Answering speeding up means act three needs re-teaching

09 Tutor FAQ

Must students memorise the three laws word for word?

No. They must be able to explain zero net force means unchanged velocity, and net force equals mass times acceleration, then use both correctly in questions.

Do S3 students need projectile or circular motion?

No. S3 Science only covers straight-line speeding up and slowing down. Projectiles and circular motion belong to S4 Physics.

Is 45 minutes enough for graph questions?

Yes, if the graph has one speeding-up section and one steady section. If half the class cannot read the slope, cut practice to two questions and spend the time on the graph.

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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.