AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
(HT only) Students should be able to: describe examples of the forces…
Full specification wording
Resultant forces: (HT only) Students should be able to: describe examples of the forces acting on an isolated object or system use free body diagrams to describe qualitatively examples where several forces lead to a resultant force on an object, including balanced forces when the resultant force is zero.
Higher tier only.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 4 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Higher tier only. A free body diagram isolates one object and shows only the forces acting on that object, each drawn as a labelled arrow starting on the object and pointing the way the force acts, with the length showing its relative size. Forces the object exerts on other things do not appear. A book on a table gets weight down and normal contact force up; a car at constant speed gets driving force forwards, drag backwards, weight down and normal contact force up. When opposing arrows are equal in length, the forces are balanced and the resultant force is zero, so the object stays at rest or keeps a constant velocity.
What earns the marks
- A free body diagram showing the chosen object alone with every force acting on it.
- Each force drawn as a labelled arrow from the object, with the length showing its relative size.
- Describing a balanced case, where the arrows cancel and the resultant force is zero.
- Linking a zero resultant force to the object remaining at rest or moving at constant velocity.
Common mistakes and how to put them right
Drawing forces that the object exerts on other objects instead of forces acting on it.
Drawing all arrows the same length when the object is accelerating.
Including invented forces such as 'the force of motion' or momentum.
Leaving arrows unlabelled, so the examiner cannot identify the forces.
Omitting one real force, most often the normal contact force or drag.
Exam tips
- Name the object your diagram is about before you draw; a diagram of two objects at once cannot be a free body diagram.
- Make the arrow lengths tell the story: equal for balanced, clearly unequal for accelerating.
Try the lesson
The start of the Resultant forces: (HT only) Students should be able to: describe examples of the forces acting on an isolated object or system use free body diagrams to describe qualitatively examples where several forces lead to a resultant force on an object, including balanced forces when the resultant force is zero. lesson, made from checked teaching notes for this specification point.
Have a go first
A book rests on a horizontal table. Which description of a free body diagram correctly represents the forces acting on the book?
Listen
Higher tier only. A free body diagram isolates one object and shows only the forces acting on that object, each drawn as a labelled arrow starting on the object and pointing the way the force acts, with the length showing its relative size. Forces the object exerts on other things do not appear. A book on a table gets weight down and normal contact force up; a car at constant speed gets driving force forwards, drag backwards, weight down and normal contact force up. When opposing arrows are equal in length, the forces are balanced and the resultant force is zero, so the object stays at rest or keeps a constant velocity. A strong answer earns marks for these points. a free body diagram showing the chosen object alone with every force acting on it. each force drawn as a labelled arrow from the object, with the length showing its relative size. describing a balanced case, where the arrows cancel and the resultant force is zero. linking a zero resultant force to the object remaining at rest or moving at constant velocity. Exam tip: Name the object your diagram is about before you draw; a diagram of two objects at once cannot be a free body diagram.
See it
Key words
Say what each word means before you turn the card.
free body diagram
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arrow length shows size
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balanced forces
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resultant force
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See every definition
- free body diagram
- A diagram that isolates one object and shows only the forces acting on that object, each drawn as a labelled arrow starting on the object.
- arrow length shows size
- Each force is drawn as a labelled arrow pointing the way the force acts, with the length showing its relative size.
- balanced forces
- Opposing arrows are equal in length, so the forces are balanced and the resultant force is zero.
- resultant force
- When several forces lead to a resultant force on an object, a zero resultant force means the object stays at rest or keeps a constant velocity.
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