S1 Science | Forces, Motion and Simple Machines

S1 Science | Forces, Motion and Simple Machines – Tutor Prep Guide (45 min)

2026-11-13

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Almost every student can say that a force is a push or a pull, yet few can explain why a book resting on a desk still has forces acting on it. Three cheap demonstrations and one speed formula turn force, inertia and friction into a single story, and the lever and the ramp show what a machine really trades away.

Objectives — Students can: (1) define a force as a push or a pull with size and direction, separate contact forces (friction, normal force, tension) from non-contact forces (gravity, magnetism), and state that a 5 kg bag weighs 50 N on Earth using g = 10 N/kg but only about 8.3 N on the Moon while its mass stays 5 kg; (2) compute speed as distance divided by time, so 100 m in 20 s is 5 m/s or 18 km/h, and state Newton first law in plain words – with no resultant force an object stays still or keeps moving in a straight line at constant speed, which is why braking and turning both need a force; (3) use effort times effort arm equals load times load arm to show that a 300 N load 0.5 m from the pivot needs only 100 N applied 1.5 m from the pivot, and explain that lifting a 600 N box by 2 m needs 1,200 J whether it is lifted straight up or pushed 4 m up a ramp at only 300 N, so a machine saves force but never saves work.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Mass and weight	How heavy is a 1 kg book on Earth, and how do mass and weight differ?	About 10 N, mass stays the same while weight depends on gravity; treating them as one idea means reteach first.
Everyday sense of speed	Is 100 m in 20 s fast, and roughly how many km/h is that?	5 m/s, about 18 km/h; failing this means revising division and the 3.6 factor.
Energy and work	When you lift a box, where does the energy go?	Into gravitational potential energy; answering that it disappears means energy conservation needs revisiting.

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Hook	Leave a book on the desk and ask whether it has any forces on it, then drop it and ask what changed	Groups write two answers to check later
5-14 min	What a force is	Pushes and pulls with size and direction, contact versus non-contact, 5 kg gives 50 N and about 8.3 N on the Moon	Two-column table with two examples each
14-23 min	Inertia and friction	Coin-and-card trick for inertia, then push the same book on a desk and on a towel	One sentence on why braking needs a force
23-32 min	Speed and graphs	100 m in 20 s gives 5 m/s and 18 km/h, then read the slope of a distance-time graph	Two speed questions with units written
32-41 min	Machines trade force	Moment calculation gives 100 N, then a ramp turns 600 N into 300 N over twice the distance	Show 1,200 J and say why work is unchanged
41-45 min	Exit check	Three questions collected and marked on the spot	Three answers, each with one reason

03 Teaching script (read it out)

Scene 1: Act 1: What a force is, and weight versus mass

Start from what students already do: push a door, pull a school bag, pedal a bike. Write the answers as two columns, push and pull. A force has two parts, size and direction, so “harder” is not an answer and “20 N to the left” is. Then sort forces: a hand pushing, a rope pulling, the floor supporting and friction are all contact forces; gravity and magnetism act at a distance. That sorting decides how free-body diagrams are drawn – contact forces sit on the contact surface, weight hangs from the centre of mass. Move on to mass and weight: mass is measured in kg and is the amount of matter, unchanged anywhere; weight is the pull of gravity, measured in N, given by W = mg with g = 10 N/kg in junior science, so a 5 kg bag weighs 50 N. On the Moon g is about one sixth, so the same bag weighs about 8.3 N while its mass is still 5 kg. Students blur these constantly, so the same object has to be weighed twice in the lesson.

Students restate: mass is unchanged, weight changes with gravity; 5 kg is about 8.3 N on the Moon.

Scene 2: Act 2: Inertia, friction and speed

Balance a coin on a card over a cup and flick the card away; the coin drops straight in. Most students say “inertia” but cannot define it: with no resultant force an object keeps its state of motion. A book resting on a desk is not force-free – gravity and the normal force cancel, so the resultant is zero. Pushing a trolley needs effort because friction and air resistance oppose motion; a spacecraft with its engine off keeps going in a straight line at constant speed because of inertia. Friction always opposes motion and turns kinetic energy into heat: useful for walking, braking and holding a pen, costly in wear and wasted fuel. Then compute: 100 m in 20 s is 5 m/s, and multiplying by 3.6 gives 18 km/h. Insist on units in every answer. Finish with a distance-time graph: steeper means faster, a flat line means stationary.

Students restate: no resultant force means no change of motion; friction opposes motion; speed is distance over time.

Scene 3: Act 3: Levers, pulleys and ramps – force saved, work not saved

Do not teach “levers save force” as a slogan; teach moments. Effort times effort arm equals load times load arm. Build a lever with a ruler and a pencil: a 300 N load 0.5 m from the pivot is balanced by 100 N applied 1.5 m away, because 300 times 0.5 equals 100 times 1.5. That single line explains why pliers have short jaws and long handles. Pulleys come in two kinds: a fixed pulley only changes direction, while a movable pulley shares the load between two rope sections, halving the force but doubling the rope pulled. The ramp is the clearest case: lifting a 600 N box by 2 m needs 1,200 J, and pushing it 4 m up a ramp needs only 300 N, yet 300 times 4 is still 1,200 J. So the rule of machines is simple – force is traded for distance, and work stays the same. This is the seed of energy conservation in S3, so plant it today.

Students restate: moments balance, a movable pulley halves force, a ramp saves force but adds distance, and work is unchanged.

04 Question types and worked steps

Type	Example	Steps	Answer
Unit conversion and weight	A bag has mass 5 kg. Find its weight on Earth and on the Moon (g on Earth 10 N/kg, about one sixth on the Moon).	(1) Write W = mg (2) 5 times 10 gives 50 N (3) 50 divided by 6 is about 8.3 N (4) Add that mass is still 5 kg	50 N on Earth, about 8.3 N on the Moon, mass 5 kg in both places.
Speed calculation	A runner covers 100 m in 20 s. Find the average speed in m/s and in km/h.	(1) Write v = s divided by t (2) 100 divided by 20 gives 5 m/s (3) multiply by 3.6 for 18 km/h (4) state units	5 m/s, which is 18 km/h.
Moments and machines	A 300 N load is 0.5 m from the pivot. How much effort is needed 1.5 m from the pivot?	(1) Write effort times effort arm equals load times load arm (2) 300 times 0.5 is 150 (3) 150 divided by 1.5 is 100 N (4) Note the force is reduced three times	100 N, a force advantage of three.
Explanation	Why does a longer ramp need less force but the same work?	(1) Work is force times distance along the force (2) Raising the box 2 m always costs 1,200 J (3) A longer ramp lowers the force and raises the distance (4) Conclude that force is traded for distance	Because work is force times distance, so a longer distance means a smaller force with the same product.

Sanity check (teach this): Reasonableness check: 5 kg times 10 gives 50 N, so 5 N means g was forgotten; the Moon gives about 8.3 N while mass stays 5 kg, so “lighter mass on the Moon” is a concept error; 100 divided by 20 is 5 m/s, while 2,000 m/s means dividing the wrong way; 5 times 3.6 is 18 km/h, so 1.8 is a decimal slip; 300 times 0.5 is 150 N m and dividing by 1.5 m gives 100 N, an answer larger than the load is impossible; 600 times 2 is 1,200 J and a 4 m ramp gives 300 N, so claiming a ramp reduces work breaks conservation of energy.

05 Independent practice (3 questions)

Question	Answer	What to watch
How much does a 2 kg dumbbell weigh on Earth? (g = 10 N/kg)	2 times 10 is 20 N.	Check g was used, and the unit is N not kg.
A cyclist covers 60 m in 10 s. Find the average speed.	60 divided by 10 is 6 m/s, about 21.6 km/h.	Check the division direction and the unit.
A 400 N load sits 0.4 m from the pivot. Find the effort at 1.6 m.	400 times 0.4 is 160, and 160 divided by 1.6 is 100 N.	Check the arms were not swapped and the answer is smaller than the load.

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating mass and weight as one thing	“On the Moon gravity is weaker, so the mass is smaller too.”	Say: “Mass is how much matter there is, so 5 kg stays 5 kg on the Moon; the weight changes from 50 N to about 8.3 N because g changes.”
Believing a still object has no forces	“The book is not moving, so no forces act on it.”	Say: “Two forces act: 50 N of weight down and 50 N of normal force up. They cancel, the resultant is zero, so it stays still.”
Confusing force saved with work saved	“A ramp saves force and work.”	Say: “Lifting 2 m costs 1,200 J. On a 4 m ramp the force is only 300 N, but 300 times 4 is still 1,200 J. Less force means more distance, never less work.”
Leaving speed without a unit	“Its speed is 5.”	Say: “5 is a number, not an answer. Write 5 m/s, or 18 km/h. An answer without a unit is marked wrong.”

07 Homework (paper, section and question numbers)

Science index: redo questions 1 to 3 from this lesson and draw the free-body diagram of a book at rest — Lesson practice 1-3
Science index: in 120 words explain why a ramp needs less force than a ladder but the same work, including the 1,200 J calculation — Act 3 of this lesson

08 Exit check (last 5 minutes)

Question	Answer	Decision
What are the weights of a 5 kg object on Earth and on the Moon?	50 N on Earth and about 8.3 N on the Moon; mass is 5 kg in both places.	Answering “8.3 kg on the Moon” means mass and weight are still confused.
Does a book at rest have any forces on it?	Yes: weight down and normal force up, equal in size, so the resultant is zero.	Answering “no forces” means inertia is not yet clear.
When a machine saves force, what does it never save?	Work; the distance increases instead.	Saying only “it saves force” misses the trade.

09 Tutor FAQ

Which year is this for?

S1. S3 energy and power lessons reuse the moment and ramp calculations, and S4 physics restates them with vectors and Newton second law.

Can three acts fit in 45 minutes?

Yes. Each act runs 9 to 10 minutes, the demonstrations need only a coin, a card, a ruler and a pencil, and only one speed and one moment calculation are done in class.

Any cross-topic links?

Yes. Ratio and unit conversion in mathematics, levers and structures in design and technology, and starting or braking in PE all connect directly.

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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.