AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Newton's First Law: If the resultant force acting on an object is zero and:
This statement introduces the two cases that follow from Newton’s First Law when the resultant force is zero.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 2 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This statement introduces the two cases that follow from Newton’s First Law when the resultant force is zero. Case one: the object is stationary, so it stays stationary. Case two: the object is already moving, so it continues at the same speed in the same straight line. The key idea is that zero resultant force means no change in velocity, whether that velocity is zero or non-zero. For example, a cyclist travelling at a steady 6 m/s on a straight flat road has zero resultant force; the driving force balances friction and air resistance. If the cyclist stops pedalling, friction and air resistance become the resultant force, so the motion changes.
What earns the marks
- Identify that the statement sets out two cases: a stationary object and a moving object.
- State that a stationary object remains stationary when the resultant force is zero.
- State that a moving object continues at the same speed in a straight line when the resultant force is zero.
- Explain that zero resultant force means no change in velocity, including no change in direction.
- Apply the two cases to a named example, such as a car cruising at steady speed on a straight road.
- Recognise that a non-zero resultant force is needed to start, stop, speed up, slow down or change direction.
Common mistakes and how to put them right
Thinking a moving object with zero resultant force will slow down and stop.
Put it right by stating it continues at constant velocity.
Believing a stationary object with balanced forces will move.
Put it right by explaining that balanced forces produce no change in motion.
Treating “steady speed” and “constant velocity” as identical when direction changes.
Put it right by noting that a change in direction is a change in velocity.
Exam tips
- Write both cases explicitly: stationary stays stationary, moving keeps constant velocity.
- Use the phrase “same speed in a straight line” to cover both magnitude and direction.
- Check whether the question asks for a description or an explanation, and include the resultant force in either case.
Try the lesson
The start of the Newton's First Law: If the resultant force acting on an object is zero and: lesson, made from checked teaching notes for this specification point.
Have a go first
A book is resting on a table. The forces acting on the book add up to zero. What is the book doing?
Listen
This statement introduces the two cases that follow from Newton’s First Law when the resultant force is zero. Case one: the object is stationary, so it stays stationary. Case two: the object is already moving, so it continues at the same speed in the same straight line. The key idea is that zero resultant force means no change in velocity, whether that velocity is zero or non-zero. For example, a cyclist travelling at a steady 6 m/s on a straight flat road has zero resultant force; the driving force balances friction and air resistance. If the cyclist stops pedalling, friction and air resistance become the resultant force, so the motion changes. A strong answer earns marks for these points. Identify that the statement sets out two cases: a stationary object and a moving object. State that a stationary object remains stationary when the resultant force is zero. State that a moving object continues at the same speed in a straight line when the resultant force is zero. Explain that zero resultant force means no change in velocity, including no change in direction. A common slip: Thinking a moving object with zero resultant force will slow down and stop. Instead, stating it continues at constant velocity. Exam tip: Write both cases explicitly: stationary stays stationary, moving keeps constant velocity.
See it
Key words
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Zero resultant force
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Stationary objects
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Constant velocity
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Changes in motion
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See every definition
- Zero resultant force
- Means no change in velocity, whether that velocity is zero or non-zero.
- Stationary objects
- When the resultant force is zero, an object that is stationary stays stationary.
- Constant velocity
- A moving object with zero resultant force continues at the same speed in the same straight line.
- Changes in motion
- A non-zero resultant force is needed to start, stop, speed up, slow down or change direction.
Label the diagram
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1Box 1
2Box 2
3Box 3
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