AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Use free body diagrams to describe qualitatively examples where several…
Full specification wording
Resultant forces: 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.
A free body diagram shows one chosen object as a dot or box with arrows for every force acting on it from its surroundings.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 4 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A free body diagram shows one chosen object as a dot or box with arrows for every force acting on it from its surroundings. Arrow length represents magnitude and the arrowhead gives direction. To describe the motion qualitatively, compare the forces along each line: if the forward and backward arrows are equal, the resultant force is zero and the object stays at rest or moves at constant velocity; if one arrow is longer, there is a resultant force in that direction and the object accelerates, slows or changes direction. For example, a skydiver with weight 700 N down and drag 500 N up has a resultant force of 200 N downwards, so they accelerate downwards. Only forces on the chosen object are drawn, never forces it exerts on other things.
What earns the marks
- Draws only the forces acting on the chosen object, with each arrow starting at the object.
- Uses arrow length to represent the relative magnitude of each force and arrowhead to show direction.
- Compares opposing forces to decide whether the resultant force is zero or non-zero.
- States the direction of the resultant force when the forces are unbalanced.
- Links a zero resultant force to constant velocity or rest, and a non-zero resultant force to a change in speed or direction.
- Describes the example qualitatively in words rather than calculating a numerical resultant.
Common mistakes and how to put them right
Drawing forces that the object exerts on other bodies.
Put it right: A free body diagram shows only forces acting on the chosen object.
Treating a longer arrow as a larger mass.
Put it right: Arrow length represents force magnitude, not mass.
Assuming a moving object must have a resultant force.
Put it right: Constant velocity means the resultant force is zero.
Adding forces in different directions as if they were in the same line.
Put it right: Only forces along the same line are combined directly.
Exam tips
- Label every arrow with the force name and, if given, its value in newtons.
- State clearly whether the resultant force is zero or non-zero before describing the motion.
- Use the phrase resultant force in the direction of the longer arrow to make the direction explicit.
- Keep the diagram simple: one dot or box for the object and straight arrows for the forces.
Try the lesson
The start of the Resultant forces: 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 box rests on a table. A free body diagram shows a downward arrow labelled weight and an upward arrow labelled normal contact force. The two arrows are exactly the same length. What is the resultant force on the box?
Listen
A free body diagram shows one chosen object as a dot or box with arrows for every force acting on it from its surroundings. Arrow length represents magnitude and the arrowhead gives direction. To describe the motion qualitatively, compare the forces along each line: if the forward and backward arrows are equal, the resultant force is zero and the object stays at rest or moves at constant velocity; if one arrow is longer, there is a resultant force in that direction and the object accelerates, slows or changes direction. For example, a skydiver with weight 700 N down and drag 500 N up has a resultant force of 200 N downwards, so they accelerate downwards. Only forces on the chosen object are drawn, never forces it exerts on other things. A strong answer earns marks for these points. Draws only the forces acting on the chosen object, with each arrow starting at the object. Uses arrow length to represent the relative magnitude of each force and arrowhead to show direction. Compares opposing forces to decide whether the resultant force is zero or non-zero. States the direction of the resultant force when the forces are unbalanced. A common slip: Drawing forces that the object exerts on other bodies. Instead, a free body diagram shows only forces acting on the chosen object. Exam tip: Label every arrow with the force name and, if given, its value in newtons.
Key words
Say what each word means before you turn the card.
Free body diagram
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Arrow length and arrowhead
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Resultant force
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Balanced forces
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See every definition
- Free body diagram
- A diagram showing one chosen object as a dot or box with arrows for every force acting on it from its surroundings.
- Arrow length and arrowhead
- The length represents the magnitude of a force and the arrowhead gives its direction.
- Resultant force
- The overall force found by comparing the forces along each line; it is zero when the forward and backward arrows are equal.
- Balanced forces
- When the forward and backward arrows are equal, the resultant force is zero and the object stays at rest or moves at constant velocity.
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