S3 Science | Forces and Simple Machines: a Tutor Prep Guide | Moments, pulleys, incline efficiency and gear speed

2026-11-09

Tutor lesson-prep guides · Secondary Physics

Students hear “simple machine” and expect free force. Then they multiply force by distance and call it work. The one line that separates them: a machine changes the size and direction of a force, never the work you must do.

Objectives — By the end the student can: (1) compute friction as coefficient times normal force, 0.35 times 240 newtons equals 84 newtons, and name three ways to change it; (2) solve load times load arm equals effort times effort arm for a 480 newton load at 0.35 metres with a 1.40 metre effort arm, giving 120 newtons and a mechanical advantage of 4; (3) explain that a three-rope pulley shares the load three ways and costs three times the rope pulled; (4) find incline mechanical advantage as length over height and efficiency as output work over input work, 75 per cent, and say why efficiency never reaches 100 per cent.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Weight and mass	A 24 kilogram mass on Earth weighs how many newtons	About 240 newtons, multiply by 10
Ratio and units	How many centimetres is 0.35 metres	35 centimetres, and state the direction
Work	A 200 newton push over 3 metres does how much work	600 joules, force times displacement

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Hook	Ask why a crowbar lifts a rock so easily; collect instincts first	Write two instinctive lines to compare later
8-18 min	Friction	Pull a block, add masses, draw the normal force arrow	Compute 0.35 times 240 as 84 newtons
18-30 min	Moment balance	Teach moment as force times arm; write 480 newtons at 0.35 metres	Solve for 120 newtons of effort
30-42 min	Pulleys	Thread three ropes, count the supporting segments, pull 1.8 metres to lift 0.6	Get 120 newtons and explain the distance cost
42-52 min	Incline and efficiency	Incline 3.6 metres long, 0.9 metres high, real effort 200 newtons	Get mechanical advantage 4 and efficiency 75 per cent
52-60 min	Exit check	Mark three questions at once, name the act each error came from	Work independently and self-assess

03 Teaching script (read it out)

Scene 1: Three tools, one principle

Textbooks split levers, pulleys and inclines into three chapters, yet the student needs one principle: a machine changes the size or direction of a force, not the work.
Method: draw a 600 newton box and write that lifting it 0.9 metres directly takes 540 joules. Then write that pushing it up an ideal incline also takes 540 joules, and ask what the incline is for. The answer is less force but more distance.
Say that line three times; every later calculation is a consequence of it.

Student should repeat: a machine changes force, never the work.

Scene 2: Moments: force times arm

A moment is force times the perpendicular distance from the pivot to the line of action. Balance means load moment equals effort moment. A 480 newton load at 0.35 metres gives 168 newton metres; with a 1.40 metre effort arm the effort is 168 divided by 1.40, which is 120 newtons.
The skill is the diagram: mark the pivot first, then both arms. The arm is not the length of the bar, it is the perpendicular distance.
Mechanical advantage is effort arm over load arm, 1.40 over 0.35, which is 4.

Student should repeat: the arm is the perpendicular distance from pivot to line of action.

Scene 3: Efficiency: output over input

An incline 3.6 metres long and 0.9 metres high has a mechanical advantage of 4, so with a 600 newton load the ideal effort is 600 divided by 4, which is 150 newtons. Measured effort is 200 newtons because of friction.
Input work is 200 times 3.6, which is 720 joules; output work is 600 times 0.9, which is 540 joules; efficiency is 540 over 720, which is 75 per cent.
Have the student press the calculator and explain that the missing work became heat and sound.

Student should repeat: efficiency is output work over input work, always below 100 per cent.

04 Question types and worked steps

Type	Example	Steps	Answer
Friction	A block weighs 240 newtons, coefficient 0.35; friction while sliding	Normal force times coefficient; newtons, opposite to motion	84 newtons
Moment balance	480 newton load 0.35 metres from pivot, effort arm 1.40 metres	Load moment first, then divide by the effort arm	168 newton metres, effort 120 newtons
Pulley system	360 newton load on three supporting ropes	Load divided by rope segments; rope pulled equals height times segments	120 newtons, pull 1.8 metres
Gears	15-tooth gear at 90 rpm driving a 45-tooth gear	Tooth ratio 45 over 15 is 3; speed divided by 3	30 rpm, torque tripled

Sanity check (teach this): Sanity check: effort should be smaller than the load unless the mechanical advantage is below 1, as with a third-class lever; gaining force costs distance in the same ratio, so force times distance stays the same; efficiency lies between 0 and 100 per cent, so 120 per cent means input and output are swapped; a small gear driving a large one always slows down and increases torque.

05 Independent practice (3 questions)

Question	Answer	What to watch
600 newton load 0.20 metres from the pivot, effort arm 0.80 metres: effort and mechanical advantage	150 newtons, advantage 4	Check arms are perpendicular distances, not bar length
450 newton load on a five-rope pulley: effort, and rope pulled to lift 0.4 metres	90 newtons, 2.0 metres	Check the rope count is multiplied, and distance grows
Incline 2.4 metres long, 0.6 metres high, real effort 200 newtons, load 600 newtons: efficiency	75 per cent	Input uses real effort times incline length; output uses load times height

06 Common mistakes and what to say

Mistake	Student writes	Say this
Using the bar as the arm	The effort arm is the whole 2 metre bar	Measure from the pivot to the line of action, perpendicular. Two metres makes the effort half of what it should be.
Thinking less force means less work	The pulley saves three times the force, so it saves work too	You pull three times the distance. Machines never reduce the work you do.
Confusing efficiency with advantage	The incline efficiency is 4	Efficiency is a percentage: output work over input work. Advantage is a ratio of forces.
Assuming small drives large speeds up	15 teeth driving 45 teeth spins three times faster	More teeth means slower and stronger. To go faster, drive a small gear with a large one.

07 Homework (paper, section and question numbers)

Physics chapter index — All chapters in the subject
Redo this chapter — Recompute moments and efficiency with questions 3, 5 and 7
Exam-style long questions — See how the same moment idea is packaged

08 Exit check (last 5 minutes)

Question	Answer	Decision
480 newton load at 0.35 metres, effort arm 1.40 metres: effort	120 newtons	Answering 480 means no moments; reteach act two
Three-rope pulley lifting 0.6 metres: rope pulled	1.8 metres	Answering 0.6 means the rope count was missed; reteach act three
Input 720 joules, output 540 joules: efficiency	75 per cent	Answering 133 per cent means the fraction is inverted

09 Tutor FAQ

Must students memorise the moment formula?

Yes, but drawing the arms matters more. Load times load arm equals effort times effort arm; lost marks usually come from a wrong arm, not a forgotten formula.

How do you count pulley ropes?

Count only the segments supporting the moving pulley; ropes on fixed pulleys do not count. One segment wrong doubles the effort, so point at each one.

Can efficiency exceed 100 per cent?

No. Output work is at most input work; the difference becomes heat and sound. A result above 100 per cent means real and ideal effort were swapped.

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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.