Lesson focus: Reliable forceA push or a pull, measured in newtons (N). explanations begin by auditing the chosen object and every interaction.
Diagnose and repair force diagrams in unfamiliar situations.
Build on earlier learning
Balanced and unbalanced forcesApply resultant-force reasoning to unfamiliar contexts.
You will be successful when you can…
- Keep arrows on one object.
- Label forces by interaction.
- Correct direction errors.
- Infer motion only after resultant.
Words that carry the science
- free-body diagram
- Force diagram isolating one object.
- normal force
- Support force perpendicular to a contact surface.
- tension
- Pulling force transmitted by a taut rope or cable.
- assumption
- Condition accepted without direct evidence.
- diagnosis
- Identification of an error from evidence.
Audit object, interaction, arrow and outcome
01Choose the system
“Car”, “person” or “car plus trailer” gives different external-force sets. State the object first.
02Audit each arrow
Ask what other object causes the force, whether it acts on the chosen body, and whether direction/length fit evidence.
03Infer last
Only after summing forces should you predict acceleration. Motion alone does not reveal every force.
Repair the walker diagram
A diagram shows ground friction on a walking person pointing backward.
- Chosen object is person.
- Foot pushes ground backward.
- Ground friction on person is therefore forward.
- Reverse the arrow, then compare it with drag/other horizontal forces.
How to make the reasoning visible: Interaction reasoning determines direction.
Scenario clinic
Complete the missing audit.
| Scenario | Correct forces | Common error | Repair |
|---|---|---|---|
| Lamp at rest | Tension up; weight down | Put tension on ceiling | Keep both on lamp |
| Trolley speeding right | Drive right > drag left | Equal arrows | Lengthen drive |
| Walker accelerating | Friction forward | Friction always backward | Use slip tendency |
| Book on slope | Weight, normal, friction | Normal vertical | Normal perpendicular to slope |
Why is the normal force on a slope not vertical?
What is missing from “the car moves right, so resultant is right”?
Use the table. For a lamp hanging still, which two forces act on the lamp?
Check your understanding
Answer from memory, then check against the suggested answer.
What is free-body diagram?
What is normal force?
What is tension?
What is weight?
Which force acts up from a surface on an object resting on it?
A box slides right while slowing. What horizontal resultant direction?
Why can a force diagram omit engine forces inside a car system?
Draw or describe the forces on a trolley that is speeding up to the right.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A learner draws equal forces to the left and right on a trolley that is speeding up to the right. Evaluate the diagram.
(3)Draw a corrected force diagram on the pad.
(1)Draw your answer in the box below. Then press Show model answer and tick each point your drawing shows.
Draw with a mouse, finger or stylus.
Total for this question: 4 marks
Command word: Link the observed acceleration to the correction.